Failed the MOT on diesel emissions: how do I lower them before the retest?

Clean the fuel system and burn the soot off before you go back. A diesel fails the MOT emissions test on smoke opacity, and most of that smoke is soot from dirty injectors, a carboned EGR valve and a lazy exhaust after-treatment system, which a fuel treatment can clean if you give it 100 to 200 miles to work. It will not get you through if the engine light is on, the DPF is blocked, the turbo vanes are seized or the engine is burning its own oil, and this page shows you how to spot those before you spend £20 on a bottle.

By Phil Clements, BMW Tool RentalUpdated 16 September 2026

Why did my diesel fail the MOT on emissions?

The tester did not measure gases. On a diesel the test is how dark the smoke is.

A diesel MOT emissions test is a smoke opacity test. The tester warms the engine, puts a probe in the tailpipe and revs it to the governor a few times while a meter measures how much light the smoke blocks. The result is a number on your fail sheet and the limit depends on the age of the car. Ask for the printout if you were not given it, because the number tells you how far off you are.

Diesel smoke is soot from fuel that did not burn properly. The usual reasons on an older car are dirty injectors with a poor spray pattern, a carboned-up EGR valve, carbon in the combustion chamber, and an exhaust after-treatment system that has not been getting hot enough to clear itself. Short journeys and a long gap between services push the number up. Forte Diesel Emissions Reducer exists for exactly this mix of deposits: it cleans the fuel system, the combustion chamber and the exhaust after-treatment system in one tank.

Two other reasons a diesel smokes are out of reach of any fuel additive. Blue smoke is engine oil, and it points at valve stem seals, piston rings or a breather. A blocked DPF or a turbo with stuck vanes is a fault, not a deposit problem, and those cars usually have a warning light on. A lit engine management light is a fail on its own regardless of the smoke reading, so that fault needs fixing before any treatment is worth buying. Dose, price and the honest limits are on the Forte Diesel Emissions Reducer product page.

What should I check before buying anything?

Five minutes with the fail sheet and the bonnet up saves people buying the wrong thing. This is the order Phil goes through with customers.

Read the fail sheetFind the smoke opacity result and the limit. A car that is just over the line responds well to a clean-up and a hard drive. A car at two or three times the limit usually has a fault underneath the soot.
Look at the dashboardEngine management, DPF or glow plug lights on means a stored fault code. The MOT fails on the light anyway, and an additive cannot clear a fault. Get the code read first; a blocked DPF and a sticking turbo have their own pages on this site.
Check the oil level and the smoke colourAn over-full sump can push oil into the intake and smoke. Blue-grey smoke that smells of burnt oil is not a fuel problem. Black smoke that smells of diesel is.
Check the air filter and the service historyA choked air filter starves the engine of air and makes black smoke on its own. If the last service was a long time ago, plan the retest around a fresh oil change as well as the fuel treatment.
Ask how the car is usedShort trips, town driving and idling never let the exhaust get hot. That car gains the most from a treatment plus a long, hard drive before the retest.

A modest fail, no lights and black smoke puts you in the group where a fuel treatment is the right first move. If any of those three is wrong, read the section on when it will not help before ordering. It is often used alongside Forte Advanced Formula Diesel Treatment; the Forte Advanced Formula Diesel Treatment page explains when that pairing makes sense.

How do I lower diesel emissions for the MOT retest?

The retest is booked, so the plan has to fit the miles you can cover before it. Forte's guidance is to treat the car 100 to 200 miles before the test, and that number is the whole plan.

Add Diesel Emissions Reducer to a full tank, 100 to 200 miles out (£20). Forte Diesel Emissions Reducer goes straight into a full tank of diesel and works through the fuel system, the combustion chamber and the exhaust after-treatment system, clearing the contamination that makes the smoke dark. Forte say it reduces smoke opacity rapidly, but it needs those miles to circulate, so do not pour it in on the morning of the test and hope.
Add Advanced Formula Diesel Treatment alongside it (£16). Forte recommend running Forte Advanced Formula Diesel Treatment with the Emissions Reducer for the biggest drop. One bottle in a full tank cleans the injectors, pump, valves, combustion chamber, turbo boost valve, EGR valve and DPF or catalyst, restores the injector spray pattern and adds up to 46% lubricity. Dirty injectors make the soot, so cleaning them stops the number creeping back.
Flush the engine and change the oil and filter. Forte pair the emissions work with an engine flush and fresh oil. Advanced Formula Motor Flush goes into the old oil, idles for 15 to 20 minutes at temperature, then drains with the sludge. It is not for wet-belt engines; on those use New Generation Engine Flush instead.
Drive it properly before the retest. The last 20 to 30 minutes before you hand over the keys should be dual carriageway or motorway with the engine working, not a crawl through town. A cold, sooty exhaust is how a car that would pass on the road fails on the ramp.
What will not work. Pouring the bottle in the day before, running it on a quarter tank, or treating a car with a live fault code and expecting the light to go out.

After the retest, the same two bottles at every service keep the fuel system clean. Both suit all diesel engines; the Diesel Treatment is fine with bio-diesel to B30 and safe in wet-belt (TBIO) diesels. The Forte Advanced Formula Diesel Treatment page covers the companion product and when to add it.

When will a diesel emissions cleaner not get it through?

Phil would rather lose a £20 sale than have you buy a bottle for a car with a broken part. These are the repair-first cases.

DPF warning light or limp mode. If the particulate filter is already blocked and the car is in limp mode, a fuel treatment cannot force a regeneration the ECU is refusing, and a completely blocked filter may need removing for a manual clean. Start on the DPF warning light and limp mode page, where the regeneration blockers are listed.

P0299 underboost and turbo lag. Sticking variable vanes on the turbo stop the engine getting the air it needs, and a car short of air smokes. That is a job for Forte Diesel Turbo Cleaner, covered on the turbo lag and sticking vanes page, not the Emissions Reducer.

Blue smoke. Oil past the valve stem seals or piston rings is an oil-side problem. The blue smoke page has a five-minute test that tells you which, and the fix is Seal Conditioner for seals or a mechanical repair for rings.

A failed component. An injector that has failed electrically, a split boost hose, an EGR valve jammed on its stem, a fuel pressure fault: these set codes and need parts. The Diesel Treatment targets the deposits behind codes P0087, P0088, P0201 to P0204 and P0401, but it does not replace a part that has worn out. If in doubt, message Phil with the fail sheet number and any codes and he will tell you honestly whether a bottle is the right next step.

What is the sensible order and timing before the retest?

Working backwards from the retest date, this is the plan that uses the products the way Forte specify.

When	What to do	Why
Today	Read the fail sheet, check for warning lights, check oil level and air filter.	Rules out the faults an additive cannot touch.
200 miles before the retest	Fill the tank. Add Diesel Emissions Reducer and Advanced Formula Diesel Treatment.	Forte's 100 to 200 mile window starts here.
Same week	Motor Flush into the old oil, 15 to 20 minutes idling, drain, new filter, fresh oil.	Removes the sludge and lets the engine run clean.
Every drive until the test	Use the car normally but include some faster running.	Circulates the treatment and gets the exhaust hot.
Retest day	20 to 30 minutes of motorway or dual carriageway driving straight before the test.	Arrive hot with the soot burned off.

Ordered together, the two bottles come to £35 or more and ship free in the UK, so if the retest is in a week order today and it will be with you in time for the first tank. Petrol car instead? The test is different (CO, HC and lambda rather than smoke) and so is the product. Read the failed MOT on petrol emissions page.

Forte Diesel Emissions Reducer

The diesel pre-MOT treatment. Goes into a full tank 100 to 200 miles before the test and cleans the fuel system, combustion chamber and exhaust after-treatment system to bring smoke opacity down. Will not clear a fault code or unblock a seized DPF. 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 Diesel Emissions Reducer Full product page

Forte Advanced Formula Diesel Treatment

The injector cleaner Forte recommend alongside the Emissions Reducer. One bottle in a full tank cleans ten components from the tank to the DPF and restores the injector spray pattern. Safe with bio-diesel to B30 and in wet-belt diesels. 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.

£16Free UK delivery on £35+ orders

Order Diesel Treatment Full product page

Every question customers ask about a diesel emissions fail

How do I lower diesel emissions for the MOT?

Clean the fuel system and get the exhaust hot. Add Forte Diesel Emissions Reducer to a full tank 100 to 200 miles before the test, ideally with Advanced Formula Diesel Treatment, change the oil and filter, and drive hard on a fast road for 20 to 30 minutes straight before the retest.

How do I reduce diesel emissions for an MOT if the retest is in a few days?

Order today, because UK delivery is quick (and free on orders of £35 or more). Fill the tank, add the bottle and cover as many of the 100 to 200 miles as you can before the test, with a hard run on the way there. If you can only manage a handful of miles, a car well over the limit may need the retest moving.

What does Forte Diesel Emission Reducer actually do?

It is a fuel treatment that clears contamination from three areas: the complete diesel fuel system, the combustion chamber and the exhaust after-treatment system. Cleaner combustion means less raw soot in the exhaust, which is what the smoke opacity meter measures.

Does a diesel emissions reducer actually work?

For a car that failed because of deposits, yes: it is designed to reduce smoke opacity rapidly once it has had 100 to 200 miles to circulate. It does not work on smoke from a blocked DPF, seized turbo vanes, a failed injector or burning oil, because none of those is a deposit.

What is the best diesel emissions reducer?

Judge one on whether the maker states a timing window, whether it cleans the after-treatment system as well as the injectors, whether it is safe in every diesel, and whether the seller tells you when it will not work. Forte Diesel Emissions Reducer meets all four.

How long before the MOT should I add it?

100 to 200 miles before the test, into a full tank. It needs to circulate 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 tank of diesel, so fill up first, then add it. The Diesel Treatment is one complete bottle per full tank as well.

Can I use it in a car with a DPF?

Yes. It is suitable for all diesel engines and it cleans the exhaust after-treatment system, which includes the DPF and catalyst. If the DPF is already blocked and the warning light is on, use Forte DPF Cleaner and Regenerator and deal with that first.

Will it clear the engine management light?

No. A lit engine management light is a stored fault and an MOT fail in its own right. Get the code read. If it is deposit-related (the Diesel Treatment targets P0087, P0088, P0201 to P0204 and P0401) cleaning can stop it returning after a reset, but no additive clears a code or replaces a failed part.

Should I use Advanced Formula Diesel Treatment as well?

Forte recommend it for maximum emissions reduction, and it makes sense: the Emissions Reducer clears the soot that is there, the Diesel Treatment cleans the injectors that are making it. Together they are £36 delivered in the UK.

Do I need an oil change too?

Forte's guidance is to pair the emissions treatment with an engine flush and fresh oil and filter. Advanced Formula Motor Flush takes 15 to 20 minutes of idling before the drain. Not for wet-belt engines; New Generation Engine Flush is the wet-belt-safe one.

Is it safe for all diesel engines?

Forte state the Diesel Emissions Reducer is suitable and safe for all diesel engines. The Advanced Formula Diesel Treatment is compatible with bio-diesel blends up to B30 and verified safe in wet-belt (TBIO) diesels.

My diesel car failed the MOT on emissions but the smoke is blue, will this help?

No. Blue smoke is engine oil, not fuel, so the fix is on the oil side: Forte Seal Conditioner for hardened valve stem seals, or a mechanical repair for piston rings. The blue smoke page has a five-minute test to tell the two apart.

What about a petrol car that failed on emissions?

Different test, different product. Petrol cars are measured on CO, HC and lambda, and the treatment is Forte Petrol Emissions Reducer, which uses PEA to clean the catalytic converter and oxygen sensors. There is a separate page on this site for a petrol emissions fail.

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.