- November 1, 2026
- Posted by: editor
- Category: RECD & Emission Control Devices
RECD catalyst poisoning is the chemical deactivation of the precious-metal sites that let a catalysed retrofit emission control device oxidise CO, hydrocarbons and NO at modest exhaust temperatures. Something in the exhaust, usually sulphur from fuel or phosphorus and zinc from burnt lubricating oil, bonds to or reacts with those sites so they stop working. It is different from masking (a physical layer such as ash or oily soot covering a healthy catalyst) and from thermal sintering (heat making the metal particles grow so there are fewer active sites). All three make the catalyst look “lazy”, but the causes, the tests and the cures are not the same.
If your catalysed RECD has started to regenerate more slowly, if the diesel smell is back, or if differential pressure (ΔP) is climbing faster than it did in the first year, treat it like a patient with new symptoms. This article is laid out as a case file: what you are seeing, who the suspects are, which tests separate them, what treatment is realistic, and how to stop it happening again.
RECD designs in India vary: some coat the filter, some add a separate oxidation catalyst stage, some have no catalyst at all. Everything below applies only where your design includes a catalyst; if unsure, ask your supplier.
The patient history: why catalysts in DG exhaust are exposed
A catalyst on a DG set may see diesel from several suppliers, a 15-year-old engine, long idle exercise runs and oil top-ups of whatever grade was in the store. That is a hard life for a thin layer of platinum or palladium.
The catalyst’s job in a filter-type RECD is usually twofold. It oxidises CO and hydrocarbons once the exhaust is above its light-off temperature, and it converts part of the NO into NO2, which then oxidises trapped soot at lower temperatures than oxygen alone can manage. Total NOx is essentially unchanged by this; the catalyst only shifts the NO/NO2 split. Lose catalyst activity and you lose both jobs at once, which is why the symptoms tend to arrive together.
Symptoms: what operators actually notice
Poisoning rarely triggers an alarm. It creeps in over hundreds of running hours. The commonly reported signs of catalyst poisoning in a catalysed RECD are:
- Passive regeneration slows. ΔP that used to level off at a stable value during long runs now keeps rising.
- ΔP climbs faster at the same load. Compare like with like: a 60% load run in March against a 60% load run in September.
- CO and hydrocarbon odour returns, especially during light-load running, exercise runs and the first minutes after start.
- NO2 at the outlet drops, if anyone measures it. A healthy oxidation catalyst raises the NO2 share; a deactivated one raises it less.
- Cleaning or active regeneration (where the design has it) is needed more often than the service schedule assumed.
- The temperature rise across the catalyst, which comes from the heat of oxidising CO and HC, shrinks or appears only at higher loads.
None of these symptoms is unique to poisoning, so rule out the look-alikes first.
| Symptom | Could be catalyst poisoning | Could also be |
|---|---|---|
| ΔP rising faster at same load | Yes, less NO2 for passive soot oxidation | Ash build-up, more soot from a worn engine, sensor line blockage, lower average load |
| Diesel odour returns | Yes, CO/HC slipping through | Leaking flange or gasket upstream of the RECD, wet stacking, injector fault, odour from a different source |
| Slower regeneration | Yes | Lower exhaust temperatures from changed duty cycle, ash layer separating soot from catalyst |
| Smaller temperature rise across catalyst | Yes, fewer active sites | Thermocouple drift, lower engine-out CO/HC (a good thing), heat loss through damaged insulation |
| Outlet NO2 lower than before | Yes | Different load or temperature on test day, analyser calibration, sampling point change |
| More frequent cleaning | Yes | Higher oil consumption adding ash, more running hours than planned |
One symptom is a reason to look closer. Two or three together, with load and running hours unchanged, point more firmly towards lost catalyst activity.
The suspects behind RECD catalyst poisoning
Suspect 1: sulphur from fuel
Sulphur in diesel burns to SO2. Over a precious-metal catalyst, part of that is oxidised to SO3, which can form sulphates on the washcoat and compete with CO, HC and NO for active sites. This is the classic sulphur poisoning DOC mechanism. The catalyst still has its metal, but less of it is available, so light-off creeps up and NO-to-NO2 conversion falls.
DieselNet’s overview of DOC deactivation describes sulphur poisoning as temporary or reversible, partially or fully, with sulphur spread fairly evenly along the catalyst length. The catch is that “reversible” usually means holding the catalyst hot for long enough to release the sulphur, which a lightly loaded DG set may never do.
BS-VI retail diesel in India is ultra-low sulphur, so this suspect is less common than it was, but old tank stock, unofficial sources or industrial-grade fuels can carry far more. Our article on how ultra-low sulphur diesel helps emission control explains why fuel quality matters for every type of aftertreatment.
Fingerprint: activity loss that tracks a change in fuel source, with partial recovery after sustained high-load running.
Suspect 2: phosphorus and zinc from lubricating oil
Most diesel engine oils carry zinc dialkyldithiophosphate (ZDDP) anti-wear additives plus calcium and magnesium detergents. Oil that slips past worn rings, valve guides or turbocharger seals sends these elements down the exhaust.
Phosphorus is the most troublesome oil-derived poison. Published research shows it tends to deposit at the catalyst inlet and in the outer layer of the washcoat, forming phosphates and a glassy, glaze-like film with zinc. It blocks pores, reduces surface area and raises light-off temperatures for CO and HC. NO oxidation, which is what drives passive soot burning, is often hit particularly hard. Unlike sulphur, phosphorus poisoning is generally regarded as permanent; ordinary thermal treatment does not remove it.
Fingerprint: activity loss concentrated at the front of the catalyst, an engine with known or suspected oil consumption, blue smoke history, and no recovery after high-temperature runs. If you have seen blue haze from the stack, read why a generator emits blue smoke and treat that as a lead.
Suspect 3: calcium and magnesium ash (masking, not true poisoning)
Calcium and magnesium from oil detergents form incombustible ash, mostly as sulphates, phosphates and oxides. DieselNet’s page on filter ash notes that on a catalysed filter, an ash layer can physically separate soot from the catalyst coating, lengthening the path for NO2 to reach the soot and making regeneration less effective.
Strictly, this is masking or fouling. The catalyst underneath may be healthy. That distinction matters because ash can be cleaned out, while a phosphorus glaze cannot. How much ash a design tolerates before it needs servicing, and how easy it is to get at, varies a great deal; our guide to comparing RECD designs by ash capacity and service access covers what to ask.
Fingerprint: gradual, steady ΔP rise that is not cleared by regeneration, plus normal or near-normal catalyst light-off once the filter is cleaned.
Suspect 4: silicon
Silicon is a known catalyst poison in many exhaust applications. Possible sources around a DG set include silicone-based sealants used on intake or exhaust joints, silicone sprays and aerosol lubricants used near the air intake, some antifoam additives, and silicate-containing coolant if it leaks into the cylinders.
Fingerprint: activity loss after engine work where sealant was used. Silicon is usually confirmed only by lab analysis.
Suspect 5: heavy metals and lead from contaminated fuel
Contaminated or adulterated fuel and waste-oil blending can introduce metals. Some aftermarket fuel additives also contain metallic compounds; our article on whether diesel fuel additives can replace an RECD is worth reading before anyone doses the tank. Metals such as lead are severe, largely irreversible poisons for precious-metal catalysts.
Fingerprint: a sudden, step-change loss of activity after a specific fuel delivery or additive trial.
Suspect 6: alkali metals (sodium, potassium) from biodiesel
Biodiesel can carry trace sodium and potassium from its production process, along with calcium and magnesium. A US Department of Energy-funded study on fuel metal impurities and aftertreatment durability aged complete DOC, DPF and SCR systems on B20 blends doped with these metals at the maximum levels permitted for B100 in the US standard, and found sodium and potassium penetrated catalyst washcoats while calcium stayed largely on the surface.
If your site uses or is considering blends, read our guide to running biodiesel blends on an RECD-equipped generator and ask your supplier about compatibility before switching.
Fingerprint: activity loss after a switch to blended fuel; elevated Na or K in a lab analysis.
Suspect 7: coolant and glycol
A leaking head gasket, cracked head or failed oil cooler can send coolant into the cylinders. Glycol itself burns, but corrosion inhibitors in many coolants contain phosphorus, silicon or boron compounds, which then reach the catalyst. White, sweet-smelling exhaust and unexplained coolant loss are the giveaways; see why a generator emits white smoke.
Fingerprint: coolant loss, white smoke, sudden onset after an overheating event.
The look-alike: thermal sintering
Thermal ageing vs poisoning is the most common confusion here. Sintering happens when the catalyst is held at very high temperatures: the fine precious-metal particles migrate and merge into larger ones, and the washcoat loses surface area. Fewer exposed metal atoms means fewer active sites, so light-off moves upward just as it does with poisoning.
The usual causes in an RECD are an uncontrolled soot burn in an overloaded filter, unburnt fuel reaching the catalyst from an injector or governor fault, or long periods of very high exhaust temperature. Sintering is permanent. No cleaning or chemical treatment brings back the original particle size.
Fingerprint: activity loss after a high-temperature event (a filter that “glowed”, a cracked core, a melted gasket), uniform along the catalyst, no unusual elements in lab analysis.
Suspects at a glance
| Suspect | Main source | Mechanism | Reversible? | Where it tends to sit |
|---|---|---|---|---|
| Sulphur | Fuel, some from oil | Sulphates compete for sites | Often partly, with sustained high temperature | Spread along the catalyst |
| Phosphorus and zinc | Burnt lube oil (ZDDP), some coolants | Phosphate glaze, pore blocking | Generally no | Inlet face, outer washcoat |
| Calcium and magnesium | Oil detergents | Ash masking | Yes, by cleaning | Filter channels, over coating |
| Silicon | Sealants, sprays, coolant, dust | Surface coating | Generally no | Inlet region |
| Lead and heavy metals | Contaminated fuel, some additives | Alloying or blocking sites | Generally no | Varies |
| Sodium and potassium | Biodiesel impurities | Washcoat penetration | Generally no | Within washcoat |
| Thermal sintering | Over-temperature events | Metal particle growth | No | Uniform, or at hottest zone |
The tests: how to narrow down the suspect
Before any physical inspection of the RECD, isolate the DG set and follow lockout/tagout; OSHA’s overview of controlling hazardous energy is a useful general reference. Let the exhaust system cool fully, because RECD housings stay hot long after shutdown, and use rated lifting gear for any core removal.
Test 1: engine oil consumption
Record oil top-ups against running hours for a few weeks and compare with the engine OEM’s acceptable figure. Rising consumption is the strongest pointer towards phosphorus and zinc poisoning and faster ash loading. Our article on engine oil consumption and RECD suitability explains how to log it properly.
Test 2: fuel sulphur certificate and fuel history
Ask your fuel supplier for the delivery test certificate, and note every change of supplier, additive trial or blended fuel use around the time symptoms began.
Test 3: used oil analysis
A used oil sample sent to a competent lab can report wear metals and additive elements, including phosphorus, zinc, calcium and magnesium, as well as signs of coolant (glycol, sodium, potassium, boron, silicon) and fuel dilution. Coolant markers strongly support Suspect 7.
Test 4: ΔP trend at matched load
Plot ΔP against running hours, but only for runs at similar load and similar exhaust temperature. Mixing a 30% load exercise run with a 75% load outage run produces noise, not evidence; the reasons are set out in our guide on why load must be matched in RECD comparison tests. A steadily rising floor after each regeneration suggests ash. A rising slope during each run, with the floor unchanged, suggests slower soot oxidation, which fits loss of catalyst activity.
Test 5: inlet and outlet temperature light-off comparison
Where the RECD has inlet and outlet thermocouples, compare the temperature rise across the catalyst at several loads. A healthy catalyst adds a small exotherm once the inlet passes light-off. Log the inlet temperature at which it first appears and repeat every few hundred hours; a rising value is a reasonable field proxy for a light-off shift. Check the thermocouples against a calibrated reference first.
Test 6: CO and HC before and after the catalyst
A portable gas analyser upstream and downstream of the catalyst, at the same steady load, shows conversion directly. Test two or three loads, including your typical light load. Our article on CO and unburned hydrocarbons in RECD systems explains what conversion looks like across a duty cycle and why light-load results are always weaker.
Test 7: lab analysis of a catalyst sample
If the field tests point to poisoning and a replacement decision is coming, ask your supplier whether they can arrange laboratory analysis of a core sample or a small cut piece. Surface analysis can show which elements are present and where they sit (front face or throughout), and activity testing on a sample can confirm light-off. This is destructive and costly, so it suits fleets, warranty disputes or repeat failures.
Reading the results together
| Evidence pattern | Most likely diagnosis |
|---|---|
| High oil consumption, high P and Zn in oil, loss worst at inlet, no recovery after hot runs | Phosphorus and zinc poisoning |
| Fuel source changed, partial recovery after sustained high-load running | Sulphur poisoning |
| ΔP floor rising, light-off normal after cleaning | Ash masking |
| Coolant loss, glycol or boron in oil | Coolant-derived poisoning plus engine repair needed |
| History of a hot event, cracked or discoloured core, clean lab analysis | Thermal sintering |
| Sudden loss after one fuel delivery or additive | Fuel contamination |
Illustrative worked example: what a light-off shift costs you
This example is illustrative. The temperatures and load profile are invented to show the arithmetic, not taken from any specific product or site.
A DG set’s catalyst had a CO light-off temperature of about 200 °C at commissioning. Over 2,000 running hours, field checks showed the first exotherm moving up to about 260 °C. The site runs the set roughly 600 hours a year with this profile:
| Operating mode | Share of hours | Typical catalyst inlet temperature | Below 200 °C? | Below 260 °C? |
|---|---|---|---|---|
| Weekly idle exercise | 10% | 150 °C | Yes | Yes |
| 25% load | 20% | 190 °C | Yes | Yes |
| 40% load | 25% | 240 °C | No | Yes |
| 60% load | 30% | 320 °C | No | No |
| 80% load | 15% | 400 °C | No | No |
| Total | 100% |
Before (light-off 200 °C): the modes below light-off are idle exercise and 25% load. 10% + 20% = 30% of operating hours. 30% × 600 h = 180 hours a year below light-off.
After (light-off 260 °C): the 40% load mode now also falls below light-off. 10% + 20% + 25% = 55% of operating hours. 55% × 600 h = 330 hours a year below light-off.
Change: 330 − 180 = 150 extra hours a year in which the catalyst does little for CO and HC and produces little NO2 for passive soot oxidation. Relative to before, that is 150 ÷ 180 ≈ 0.83, an increase of about 83%.
A 60 °C shift sounds small, but because many DG sets spend much of their life at 30–45% load, it can move a large block of hours across the line. A site running mostly at 60–80% load would barely notice the same shift. That is why your load logs belong in any poisoning investigation; see using DG load logs to assess RECD suitability.
Treatment: what can be recovered and what cannot
Be realistic: catalyst deactivation in a DPF or DOC is often partly reversible and partly permanent, and most real cases are a mix.
Recoverable, at least partly
Sulphur poisoning. Sustained operation at higher exhaust temperatures can release adsorbed sulphur species and restore some activity. How hot, for how long, and whether it is safe for your particular filter loading are questions for your RECD supplier, not for guesswork. Running a heavily soot-loaded filter hard to “burn off sulphur” risks an uncontrolled soot burn and thermal damage, turning a reversible problem into a permanent one. Fix the fuel source first, or the problem returns.
Ash masking. Ash is removed by off-line cleaning using the method your supplier approves. If the catalyst under the ash is healthy, activity returns once the filter is cleaned. Dispose of removed ash as your SPCB and an authorised handler advise.
Light oily or soot fouling. A face coated with wet soot from light-load running can often be cleared by a controlled regeneration or cleaning procedure, again per supplier instructions. See choosing an RECD for generators running at low load for why this recurs.
Not recoverable in practice
Phosphorus and zinc glaze, silicon, lead and alkali penetration. These are generally treated as permanent. Cleaning may remove loose ash on top, but the bonded layer remains.
Thermal sintering. Permanent. The precious metal has physically changed form.
When the catalyst is beyond recovery, you choose between core and full assembly replacement; our comparison of filter core vs complete assembly replacement sets out the trade-offs. Check warranty terms, which often exclude damage from out-of-specification fuel or oil.
Never bypass or remove the RECD to keep the DG running. That is a compliance risk that can lead to sealing and penalties, and it does not fix the engine fault behind the poisoning.
Treat the cause, not only the catalyst
A new catalyst on an engine that still burns oil will be poisoned again. Before fitting any replacement core, close out the engine work: rings, valve stem seals, turbocharger seals, head gasket, injectors.
Prevention: a written plan, not good intentions
Most RECD catalyst poisoning is preventable, cheaply.
Fuel
- Buy diesel only from reliable sources and keep the delivery documents on file.
- Avoid unofficial fuel, blended waste oil and metal-based additives unless your RECD supplier has approved them in writing.
- Drain water and sludge from tanks on schedule.
- Get supplier approval before trialling biodiesel blends.
Oil
- Use the oil grade the engine OEM specifies. Ask the engine OEM whether a lower-ash, lower-phosphorus formulation (often called low-SAPS: low sulphated ash, phosphorus and sulphur) is compatible with your engine. Do not switch on your own; older engines may need the additive protection of their original specification.
- Do not top up with mixed grades from the store.
- Track oil consumption monthly and act on any rising trend.
Engine health
- Fix oil burners promptly. Blue smoke, oily exhaust joints and rising top-ups are early warnings.
- Watch coolant level; investigate any unexplained loss.
- Avoid prolonged idle and very light load where possible, so the catalyst spends more time above light-off.
Around the RECD
- Ban silicone sprays and aerosol lubricants near the engine air intake.
- Ask your RECD supplier which sealants are acceptable on joints upstream of the catalyst.
- Watch for over-temperature events: abnormal glow, discoloured housing paint, burnt insulation.
Records
Keep one log covering running hours, load, ΔP, inlet and outlet temperatures, oil top-ups, fuel deliveries, oil analysis and cleaning dates. That log turns a guess into a diagnosis. Our sibling article on reading an RECD case study explains why long-term records beat a single commissioning test. Routine checks from the RECD maintenance guide fit naturally alongside this log.
How poisoning interacts with compliance
Under the CPCB procedure for RECDs on in-use DG engines up to 800 kW, devices are type approved for at least 70% particulate matter capture. A poisoned catalyst does not necessarily stop the filter trapping soot, but slower regeneration means higher back-pressure, more cleaning and, if neglected, a risk of filter damage. Returning odour can also prompt complaints. Confirm any test or reporting obligations with your SPCB and consent conditions.
A catalysed RECD is never a reason to relax exhaust safety. No RECD fully removes CO or NOx, so exhaust must always discharge outdoors through a properly designed stack, never into an occupied space.
For a broader look at how catalysts change daily operation, see our earlier comparison of catalysed and uncatalysed RECD filters.
FAQ
What are the main signs of catalyst poisoning in an RECD?
Slower regeneration, faster ΔP rise at the same load, returning odour at light load, a smaller temperature rise across the catalyst and lower outlet NO2. Rule out ash, sensor faults and duty cycle changes first.
Is sulphur poisoning of a DOC reversible?
Often partly. Sustained high-temperature operation can release adsorbed sulphur and restore some activity, but the safe procedure depends on your RECD design and filter loading, so ask your supplier first. Switching to reliable ultra-low sulphur fuel prevents recurrence.
Can engine oil poison an RECD catalyst?
Yes. Phosphorus and zinc from anti-wear additives in burnt oil form a glaze-like deposit, mainly at the catalyst inlet, that is generally permanent. An engine with high oil consumption is the biggest risk factor.
Should I switch my DG set to low-SAPS engine oil?
Only if the engine OEM confirms it is compatible. Low-ash, low-phosphorus oils reduce poisoning and ash loading, but some older engines need their original specification for wear protection.
What is the difference between thermal ageing and poisoning?
Poisoning is chemical: a contaminant blocks or reacts with active sites. Thermal ageing (sintering) is physical: very high temperatures make the metal particles grow so fewer sites are exposed. Both raise light-off temperature, and both are usually permanent except for sulphur.
Can a poisoned catalyst be cleaned?
Ash and loose deposits can be cleaned off, which helps if masking was the problem. Bonded poisons such as phosphorus, silicon, lead and alkali metals generally cannot.
Need a Second Opinion on a Slowing RECD?
If your catalysed RECD shows these symptoms, Aceget’s team can help you work through the evidence before anyone orders a replacement core. Aceget’s product page describes its RECD as using a silicon carbide ceramic filter with a precious-metal coating, an oxidation catalyst stage and NO2-assisted regeneration, so we understand why catalyst health matters for day-to-day operation. Browse the Aceget RECD range, review our certifications, or contact us with your DG details and recent logs.