Failed the MOT on petrol emissions: what do CO, HC and the cat mean, and how do I pass the retest?

High CO or HC on a petrol MOT nearly always means the catalytic converter is not doing its job, either because it is coated in deposits or because it is being fed a bad mixture. If the engine runs cleanly and the cat is simply dirty, a PEA fuel treatment added 100 to 200 miles before the retest can clean the cat and oxygen sensors and bring the numbers down. If the engine light is on, the car misfires, it burns oil, or the cat is physically broken, no bottle will get it through and this page tells you how to spot each of those first.

By Phil Clements, BMW Tool RentalUpdated 16 September 2026

Why did my petrol car fail the MOT on emissions?

The printout has three numbers on it. Each one points at a different cause.

A petrol MOT emissions test puts a gas analyser in the tailpipe with the engine warm, at fast idle and at natural idle. It reads CO (carbon monoxide), HC (hydrocarbons) and lambda, which is the air-fuel ratio the engine is running at. The limits depend on the age of the car. Ask for the printout if you were not handed it, because which number failed tells you where to look.

High CO means the mixture is rich or the catalytic converter is not converting. High HC means unburned fuel is getting through: a misfire, a weak spark, a leaking injector, or again a cat that is not lit. Lambda out of range means the engine is not controlling its mixture, usually a tired oxygen sensor, an air leak or a fuelling fault. On an older car the common thread is deposits: carbon on the injectors and inlet valves upsetting the mixture, and contamination on the oxygen sensor and the face of the catalyst stopping them working.

That deposit case is what Forte Petrol Emissions Reducer is made for. It contains polyetheramine (PEA), a strong detergent that dissolves deposits through the fuel system, combustion chamber and exhaust, and it works before, during and after combustion so it reaches the oxygen sensors and the catalytic converter as well as the injectors. Forte associate it with the codes P0420 (catalyst efficiency) and P0171 (system too lean) for that reason.

What no fuel treatment fixes: an engine management light (a fail in its own right), a misfire from a failed coil, plug or injector, oil burning past the valve seals or rings and poisoning the cat, or a catalyst whose internals have broken up. Dose, price and the honest limits are on the Forte Petrol Emissions Reducer product page.

What should I check before buying a catalytic converter cleaner?

Phil asks every customer who messages about a petrol emissions fail the same five things. They decide whether a £20 bottle is the right purchase.

Which number failed, and by how much?Just over on CO or HC with lambda in range is the classic dirty-cat fail and responds to a clean and a hard drive. Wildly over, or lambda well out, points at a fuelling or ignition fault.
Is the engine management light on?If it is, the MOT fails on the light regardless. Get the code read. P0420 or P0430 with no misfire codes is a cat efficiency complaint that deposits can cause; P0300 to P0308 is a misfire that needs fixing first.
Was the car properly warm at the test?A catalyst has to be hot to work. A car that crawled to the test centre and sat in a queue can fail on a cold cat and pass an hour later. That is not a fault, it is a cold exhaust.
Does it use oil or blow blue smoke?Oil in the exhaust coats the catalyst and the oxygen sensor. Clean the cat all you like, it will foul again until the oil is stopped. The blue smoke page on this site has a five-minute test for valve stem seals versus rings.
When were the plugs, air filter and oil last done?Worn plugs raise HC. A blocked air filter richens the mixture and raises CO. If the service is overdue, do it as part of the retest plan rather than instead of it.

Modest fail, no light, car was warm, no oil use, service roughly up to date: that is the deposit group, and the plan below is the sensible next step.

How do I reduce petrol emissions for the MOT retest?

Forte's own timing is the whole plan: treat the car 100 to 200 miles before the test so the detergent has been through every part of the system by the time the probe goes in.

Petrol Emissions Reducer into a full tank, 100 to 200 miles out (£20). Forte Petrol Emissions Reducer is the pre-MOT product. Its PEA detergent cleans internal and external injector deposits to restore fuel flow and spray pattern, cleans the combustion chamber, and cleans the oxygen sensors and catalytic converter so they can do their job. Forte's claim is a rapid reduction in CO, HC and NOx once it has circulated.
Advanced Formula Petrol Treatment alongside it (£20). Forte recommend using Forte Advanced Formula Petrol Treatment with the Emissions Reducer. One bottle in a full tank cleans ten components from the tank to the GPF or catalyst, including the injectors, inlet valves, piston rings, lambda sensor and EGR valve, and it keeps the injector spray pattern right so the emissions stay down after the test.
Engine flush, fresh oil, new oil filter and air filter. Forte pair the emissions work with an engine flush and a fresh oil, oil filter and air filter. Advanced Formula Motor Flush goes into the old oil for 15 to 20 minutes of idling before the drain. It is not for wet-belt engines such as the Ford EcoBoost or PSA PureTech; use New Generation Engine Flush on those.
Arrive hot. Twenty to thirty minutes of fast-road driving straight before the test, not a queue in traffic. A hot catalyst converts; a lukewarm one does not.
What will not work. Adding the bottle the night before, treating a car with a live misfire, or expecting a treatment to fix a cat that rattles when you tap it.

Both products are compatible with all petrol engines including direct injection and hybrids, and with petrol and bio-petrol blends up to E20, so E10 pump fuel is fine. The Petrol Treatment is also verified safe in wet-belt engines. It is often used alongside Forte Advanced Formula Petrol Treatment; the Forte Advanced Formula Petrol Treatment page explains when that pairing makes sense.

Can a catalytic converter cleaner actually clean the cat?

Yes, when what is wrong with it is a coating. A catalyst is a honeycomb coated in precious metals, and it only converts CO and HC when the exhaust gas can reach that coating. Carbon, fuel residue and oil ash sit on the surface and insulate it. A PEA detergent carried through in the fuel dissolves those deposits as the exhaust passes over them, which is why the same product also cleans the oxygen sensors upstream and downstream of the cat.

No, when what is wrong with it is mechanical. A honeycomb that has melted from a long-running misfire, broken up and rattles, or been physically removed by a previous owner cannot be cleaned back. A cat that is repeatedly fouled by oil from worn valve stem seals or rings, or by coolant from a head gasket, will clean up and then foul again until the cause is fixed. For the oil case start with the blue smoke page and Forte Seal Conditioner.

The honest test: if the car failed on CO or HC with no engine light, no misfire and no oil use, a catalytic converter cleaner is a £20 experiment against a several-hundred-pound part, and Forte Petrol Emissions Reducer is the one Phil sells because it names the cat and the oxygen sensors as things it cleans.

A DPF and a catalytic converter are not the same thing. Petrol cars have a catalyst (newer ones a GPF too); diesels have a catalyst and a particulate filter. If yours is a diesel, the test and the product are different: read the failed MOT on diesel emissions page instead.

What is the timing plan before the retest?

Working back from the retest date, this is how to use the products the way Forte specify.

When	What to do	Why
Today	Read the fail sheet. Check for the engine light, blue smoke and service history.	Rules out the faults a treatment cannot touch.
200 miles before	Fill the tank. Add Petrol Emissions Reducer and Advanced Formula Petrol Treatment.	Forte's 100 to 200 mile window starts here.
Same week	Motor Flush, drain, new filter, fresh oil, new air filter. Plugs if they are due.	Removes the sludge and richness that raise CO and HC.
Until the test	Drive normally with some faster running.	Circulates the PEA and keeps the cat hot.
Retest day	20 to 30 minutes on a fast road straight before the test.	Arrive with a hot, converting catalyst.

UK delivery is quick (and free on orders of £35 or more), so a retest a week away is workable if you order today. If the retest is tomorrow, a warm-up drive helps, but the treatment needs its miles. The Forte Advanced Formula Petrol Treatment page covers the companion product and when to add it.

Forte Petrol Emissions Reducer

The petrol pre-MOT treatment. PEA detergent cleans the injectors, combustion chamber, oxygen sensors and catalytic converter to bring CO, HC and NOx down. Add to a full tank 100 to 200 miles before the test. Safe in all petrol engines including direct injection and hybrids. Will not fix a misfire, an oil-burning engine or a broken catalyst. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£20Free UK delivery on £35+ orders

Order Petrol Emissions Reducer Full product page

Forte Advanced Formula Petrol Treatment

The fuel system cleaner Forte recommend alongside the Emissions Reducer. One bottle in a full tank cleans ten components including the injectors, inlet valves, piston rings, lambda sensor and catalyst, and keeps the spray pattern right afterwards. Safe in direct injection, hybrid and wet-belt engines. Ships worldwide from the UK; UK delivery free on orders of £35 or more, postage and any duty shown at checkout for the USA, Canada, Australia, New Zealand and Ireland.

£20Free UK delivery on £35+ orders

Order Petrol Treatment Full product page

Every question customers ask about a petrol emissions fail

How do I reduce car emissions for the MOT on a petrol?

Clean the fuel system and the catalyst, then arrive hot. Add Forte Petrol Emissions Reducer to a full tank 100 to 200 miles before the test, ideally with Advanced Formula Petrol Treatment, do the oil, oil filter and air filter, and drive on a fast road for 20 to 30 minutes straight before the retest.

What is the best catalytic converter cleaner?

Judge one on whether it names the catalyst and oxygen sensors as things it cleans, whether it states a detergent (PEA is the one that dissolves cat deposits), whether it gives a timing window, and whether the seller says when it will not work. Forte Petrol Emissions Reducer does all four, which is why it is the one Phil stocks.

Can a catalytic converter be cleaned?

If the problem is deposits on the honeycomb, yes: a PEA fuel treatment carried through the exhaust dissolves them. If the honeycomb has melted, broken up or been removed, no. And a cat fouled by burning oil or coolant will foul again until that cause is fixed.

Is there a petrol additive to reduce emissions for the MOT?

Yes. Forte Petrol Emissions Reducer is formulated to rapidly reduce CO, HC and NOx and to help cars pass the MOT emissions test. It goes into a full tank 100 to 200 miles before the test.

Does Forte Petrol Emission Reducer actually work?

For a fail caused by deposits on the injectors, oxygen sensors or catalyst, that is exactly what it is designed to clear. It does not work on a misfire, an air leak, an oil-burning engine or a physically failed cat, because none of those is a deposit.

How long before the MOT should I add it?

100 to 200 miles before the test, into a full tank. The PEA needs to pass through the injectors, combustion chamber and exhaust to do its job. The night before is too late.

How much do I add?

Forte's instruction is to add the treatment to a full fuel tank, so fill up first. The Advanced Formula Petrol Treatment is one complete bottle per full tank as well.

Will it clear a P0420 catalyst efficiency code?

Forte associate the product with P0420 and P0171. If the code was set by a contaminated catalyst or oxygen sensor, cleaning them can stop it returning once it is reset with a scanner. No fuel treatment clears a stored code by itself, and if the cat has physically failed the code will come back.

Will it fix a P0171 lean code?

It can help if the lean reading comes from dirty injectors or a fouled oxygen sensor, which are the deposit causes Forte list. If P0171 comes from a split intake hose or a vacuum leak, that is a part, not a deposit, and it needs fixing first.

Is it safe in direct injection, hybrid and E10 cars?

Yes. Forte state the Petrol Emissions Reducer is compatible with all petrol engines including direct injection and hybrids, and with petrol and bio-petrol blends up to E20, so E10 pump fuel is fine.

Should I use Advanced Formula Petrol Treatment as well?

Forte recommend it alongside the Emissions Reducer: one clears the deposits that failed you, the other cleans ten components and keeps the injector spray pattern right so the numbers stay down. Together they are £40 delivered in the UK.

Do I need an oil change and air filter as well?

Forte's best-practice guidance is an engine flush with fresh oil, oil filter and air filter alongside the emissions treatment. A blocked air filter richens the mixture and raises CO on its own. Use Advanced Formula Motor Flush, or New Generation Engine Flush on a wet-belt engine.

My petrol car failed the MOT on emissions and the engine light is on. Will a treatment help?

Not until the light is dealt with, because a lit engine management light is an MOT fail on its own. Get the code read. If it is a deposit-related code such as P0420 the treatment is a sensible part of the fix; if it is a misfire, coil, sensor or air leak code, that part comes first.

Is a DPF the same thing as a catalytic converter?

No. A catalytic converter changes CO, HC and NOx into less harmful gases; a diesel particulate filter traps soot. Petrol cars have a catalyst; diesels have a catalyst and a DPF, and a diesel emissions fail is a smoke test with its own page and product.

Do you deliver outside the UK?

Yes. UK delivery is free on orders of £35 or more. Orders to the USA, Canada, Australia, New Zealand, Ireland and the rest of Europe are sent tracked from the UK, with postage and any duty shown at checkout before you pay. There is a dedicated page for US customers.