AdBlue warning light and P20EE: what are the crystals and how do I clear them?

AdBlue crystals are dried urea. When the exhaust is not hot enough to vaporise the fluid, usually because of short journeys, cold weather or an engine that is not burning cleanly, the AdBlue dries into white crystals on the injector, the mixer and the SCR catalyst, the catalyst stops converting NOx properly and the car logs P20EE or P204F and puts the AdBlue warning up. An additive in the AdBlue tank dissolves the existing crystals and stops new ones forming. It will not replace a failed NOx sensor, pump or injector, and it will not stop a no-restart countdown caused by an empty tank, so the checks below come first.

By Phil Clements, BMW Tool RentalUpdated 16 September 2026

What are AdBlue crystals and why has P20EE come on?

AdBlue only works as a vapour. Below the temperature it needs, it dries out where it lands.

AdBlue is a urea solution that a modern diesel injects into the exhaust just ahead of the selective catalytic reduction (SCR) catalyst. In a hot exhaust it turns to ammonia, which reacts with the nitrogen oxides on the catalyst and turns them into nitrogen and water. That is how a Euro 6 diesel meets its NOx limit, and it only works when the exhaust is hot enough to turn the liquid into gas.

When it is not, the fluid lands on metal as a liquid and dries. Forte describe the cause as exhaust temperatures too low to properly vaporise the fluid, typically from short journeys, cold weather or incomplete combustion. The result is a white crust on the AdBlue injector tip, the mixer, the SCR catalyst face and, if it is left, the pump and tank. The catalyst cannot convert what it cannot reach, the downstream NOx sensor sees the shortfall, and the car logs P20EE (SCR catalyst efficiency below threshold) or P204F (reductant system performance). Forte list those two codes as what Forte Exhaust Crystal Preventer addresses.

Left alone the crystals cause expensive failures in the SCR system, and Forte name seven components they protect: the fluid tank, level sensor, fluid pump, AdBlue injector, exhaust SCR mixer, SCR catalyst and NOx trap. Any of those, once physically broken, is a part rather than a chemistry problem, which is why the next section is about telling the two apart. Dose, price and the honest limits are on the Forte Exhaust Crystal Preventer product page.

Is it crystals, or has a part failed?

P20EE says the catalyst is under-performing. It does not say why. These checks take a quarter of an hour and decide whether a £12 bottle or a garage visit is the right next step.

Read every code, not just the headlineP20EE or P204F on their own, on a car that does short trips, is the crystal pattern. A NOx sensor code, a pump or heater code, or a level sensor code alongside it means a component has been logged as faulty, and the additive cannot fix a component.
Check the AdBlue level and any countdownA message that says the car will not restart in so many miles is the tank running down, not a crystal fault. Fill it with fresh fluid from a sealed container first. The additive goes in with the refill.
Look at the injector tip and mixer if you canOn many cars the AdBlue injector is reachable from underneath. A white crust around the tip and in the pipe beyond it is the crystal problem in plain sight.
Be honest about the drivingTen-minute school runs, town deliveries and a cold winter are the recipe. A car that does motorway miles every day and still throws P20EE is more likely to have a failed sensor or catalyst.
Check the fluid itselfAdBlue has a shelf life and degrades in heat. A jug that has sat in a hot garage for two summers, or fluid that has been topped up with anything other than AdBlue, causes its own faults and needs draining rather than treating.

Crystals visible or short-trip driving, no component codes, tank full of fresh fluid: that is the case the treatment is made for. 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 clear AdBlue crystals without stripping the exhaust?

Forte's product does two things: it chemically dissolves the crystals that are already there and it prevents new ones forming across the whole SCR system. The application is simple.

Exhaust Crystal Preventer into the AdBlue tank (£12). Forte Exhaust Crystal Preventer goes straight into the AdBlue tank at every refill. For passenger cars and light vans a 150ml bottle treats up to 20 litres of AdBlue; for HGVs and buses a 400ml bottle treats up to 60 litres. It dissolves existing crystals and prevents new ones forming across the whole SCR system.
Then get the exhaust hot. The additive works through the fluid as it is dosed, so the car needs to be driven and the SCR system needs to be injecting. A proper run with the engine working, rather than another week of short trips, gives it the chance to reach the crystals.
Keep the engine burning cleanly. Forte's technical guide recommends Advanced Formula Diesel Treatment and Advanced Formula Motor Flush at every service. Clean fuel and oil systems restore engine performance, which helps maintain the higher exhaust gas temperatures that naturally stop crystals forming. The Diesel Treatment is one bottle in a full tank; the flush is not for wet-belt engines.
Clear the code once the cause is dealt with. P20EE is a stored fault. Once the crystals are gone and the catalyst is converting again, clear it with a scanner. If it stays away over the next few hundred miles, the crystals were the cause.
What will not work. Pouring the additive into the diesel tank instead of the AdBlue tank, adding it to an empty AdBlue tank and expecting the countdown to stop, or treating a car with a dead NOx sensor and hoping the code clears.

The additive is suitable for any diesel with an AdBlue or SCR system: passenger cars, light commercials, buses and HGVs. Peugeot, Citroen and Vauxhall vans, BMW and Mercedes cars and Ford Transits all follow the same logic. The Forte Advanced Formula Diesel Treatment page covers the companion product and when to add it.

When will an AdBlue anti-crystal additive not fix it?

Phil would rather tell you now than sell you a bottle for a car that needs a part. These are the cases where the answer is a repair, with the additive keeping the repaired system clean afterwards.

A failed NOx sensor. The sensor that measures catalyst efficiency fails often and sets its own code. A dead sensor reports a fault whether the catalyst is clean or not, and no chemistry changes that.

A failed pump, heater, level sensor or injector. Forte list these among the components crystals damage, which is the point of prevention, but once one has failed electrically or mechanically it is a replacement. Crystals may well have caused it; the additive stops the next one.

An empty or contaminated tank. The no-restart countdown is the car protecting its emissions system from running without reductant. Fill it. Fluid that has been contaminated with diesel, water or the wrong product needs the tank draining and flushing.

A catalyst that has physically failed. A cracked or poisoned SCR catalyst will keep logging P20EE after every clean. If the crystals are gone and the code returns within a tank, that is the likely reason.

If your diesel also has a DPF warning light, turbo lag or has failed the MOT on smoke, those pages cover the rest of the same exhaust, and the common thread is an engine that is not burning cleanly enough to keep itself hot.

How do I stop the crystals coming back?

Treat every refill and give the exhaust heat. Forte's dosing is per refill, not a one-off, because the conditions that made the crystals (the driving pattern and the weather) do not go away.

Situation	What to do	Why
P20EE on a short-trip car, crystals visible	Fresh AdBlue plus Exhaust Crystal Preventer, long drive, clear the code	Dissolves the crust and lets the catalyst convert again
Countdown to no restart	Fill the tank with fresh fluid first, then treat	The countdown is the level, not the crystals
Component code alongside P20EE	Replace the part, then treat every refill	Chemistry cannot replace a failed sensor or pump
Van or car on town work all year	Additive at every refill, Diesel Treatment at every service	Keeps the injector clear and the exhaust hotter
HGV, coach or bus	400ml bottle per 60 litres of AdBlue	Commercial dose for the larger tank

At £12 a bottle against the cost of an AdBlue injector or a pump, treating the tank at each refill is the cheapest insurance on a modern diesel that does short journeys.

Forte Exhaust Crystal Preventer

The AdBlue additive for crystal build-up and P20EE or P204F. Goes into the AdBlue tank at every refill: 150ml treats up to 20 litres for cars and vans, 400ml up to 60 litres for HGVs. Dissolves existing crystals and prevents new ones across the whole SCR system. Will not replace a failed sensor, pump or injector. 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.

£12Free UK delivery on £35+ orders

Order Exhaust Crystal Preventer Full product page

Forte Advanced Formula Diesel Treatment

What Forte's technical guide recommends at every service alongside the crystal preventer. One bottle in a full tank cleans the injectors and combustion chamber so the engine burns cleanly and runs a hotter exhaust. 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 AdBlue crystals and P20EE

What does P20EE mean?

P20EE is the code for the SCR (AdBlue) catalyst's NOx conversion efficiency falling below its threshold. Common causes are urea crystals on the injector, mixer and catalyst from a cold exhaust, a failed NOx sensor, or a catalyst that has failed. Forte list P20EE and P204F as the codes Exhaust Crystal Preventer addresses.

P20EE on a BMW: is it the same thing?

Yes. The SCR system on a BMW diesel works the same way as any other, and short-trip driving crystallises the AdBlue the same way. Read the codes for a NOx sensor fault first, then treat the tank if it is crystals.

What is an AdBlue anti-crystal additive?

A treatment added to the AdBlue tank rather than the fuel tank. Forte Exhaust Crystal Preventer chemically dissolves existing AdBlue crystals and prevents new ones forming across the entire SCR system, protecting the tank, level sensor, pump, injector, mixer, catalyst and NOx trap.

How do I clean AdBlue crystals off the injector?

Chemically, through the fluid: add Exhaust Crystal Preventer to the AdBlue tank and drive the car so the system doses. Crystals visible on the outside of the injector tip can be wiped off, but the deposits inside the injector and along the mixer are what the additive is for.

How do I use Forte AdBlue Exhaust Crystal Preventer?

Add it directly into the AdBlue tank at every refill. A 150ml bottle treats up to 20 litres of AdBlue for cars and light vans; a 400ml bottle treats up to 60 litres for HGVs and buses. Then drive the car normally with some proper runs.

How much do I add to a car?

One 150ml bottle per 20 litres of AdBlue. Most cars take a bottle with each refill. For a commercial vehicle with a large tank, the 400ml bottle treats 60 litres.

Will it clear the AdBlue warning light?

It removes the crystals that caused a P20EE or P204F, after which the code can be cleared with a scanner and should stay off. It does not stop a no-restart countdown from a low tank, and it does not clear a code set by a failed sensor, pump or injector.

Does it fix P204F as well?

P204F is the reductant system performance code and Forte list it alongside P20EE. If crystals in the injector or mixer are the cause, dissolving them addresses it. If the pump or injector has failed, the code returns until the part is replaced.

Does AdBlue go off?

Yes. It has a shelf life and degrades in heat and sunlight, and fluid that has been contaminated causes faults of its own. Use fresh fluid from a sealed container and add the crystal preventer with it.

How often should I use it?

At every AdBlue refill. Forte's dosing is per refill because the conditions that cause crystallisation, short journeys and cold weather, are ongoing.

Is it safe for every AdBlue system?

Forte state it is suitable for any diesel vehicle equipped with an AdBlue or SCR system, including passenger cars, light commercial vehicles, buses and HGVs. The sources list no specific safety warnings beyond its intended use.

Can I use it in a Peugeot, Citroen or Vauxhall van?

Yes. PSA and Vauxhall vans on delivery work are among the most common AdBlue crystal cases because of the short-trip pattern. It is added to the AdBlue tank at each refill in the same way.

What is the best AdBlue additive?

Judge one on whether it both dissolves existing crystals and prevents new ones, whether the maker names the components it protects, whether the dose per litre of AdBlue is stated, and whether the seller says when a part is needed instead. Forte Exhaust Crystal Preventer meets all four.

Is it a cleaner or a preventer?

Both. Forte describe it as chemically dissolving existing crystals and preventing new ones forming across the SCR system. The name says preventer because the intended use is at every refill, before the damage.

Why does Forte recommend Diesel Treatment as well?

Because crystals form when the exhaust is too cool, and an engine with dirty injectors burns less cleanly and runs cooler. Forte's technical guide recommends Advanced Formula Diesel Treatment and Motor Flush at every service to restore performance and maintain the higher exhaust temperatures that naturally prevent crystals.

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.