P20EE Fault Code: What It Means and How to Fix It

Your engine warning light is on, a code reader shows P20EE, and maybe an AdBlue message has popped up on the dashboard too. This code only shows up on modern diesel cars and vans, and it is one you should not leave for long. The good news is that the cause is often something simple, such as poor AdBlue or a faulty sensor, rather than a big, expensive part. Here is what the P20EE fault code means, why it happens and what to do next.

What Does the P20EE Fault Code Mean?

P20EE stands for “SCR NOx Catalyst Efficiency Below Threshold, Bank 1”. That sounds complicated, so let us break it down.

Diesel engines make a harmful gas called NOx (nitrogen oxides). To clean it up, most diesels built from around 2015 onwards have a system called SCR, which is short for selective catalytic reduction. It works like this:

1. A small injector sprays AdBlue, a clear liquid made from water and urea, into the hot exhaust.
2. The exhaust then flows through a special catalyst.
3. Inside the catalyst, the AdBlue reacts with the NOx and turns it into harmless nitrogen and water.

Your car has NOx sensors before and after the catalyst. They check how much NOx goes in and how much comes out. If too much NOx is still coming out, the car’s computer decides the system is not working well enough. It then stores P20EE and turns on a warning light.

“Bank 1” simply means the side of the engine with cylinder number one. Most diesel cars only have one bank, so you can ignore this part.

Which Cars Get the P20EE Code?

P20EE only affects diesels with an AdBlue system. You will not see it on a petrol car. It is common on:

• Mercedes-Benz diesels with BlueTEC or BlueEFFICIENCY badges
• BMW diesels from around 2015 onwards
• Audi, Volkswagen, Skoda and Seat TDI models
• Peugeot and Citroën BlueHDi engines
• Ford EcoBlue engines, including the Transit
• Vauxhall, Renault and Nissan diesels with AdBlue

If your car has a small blue filler cap next to the diesel cap, or an AdBlue tank in the boot, it has an SCR system.

Signs Your Car Has a P20EE Fault Code

You may notice one or more of these:

• The engine warning light stays on
• An AdBlue or “emissions system fault” warning on the dashboard
• A countdown message, such as “No engine start in 500 miles”
• Higher fuel use than normal
• The car feeling flat or going into limp mode, where power is cut to protect the engine
• An MOT fail because of the warning light

The countdown message is the most important one to watch for. Many cars are built so that if the SCR fault is not fixed, the engine will not restart once the countdown reaches zero. The car keeps running until you switch it off, but you may then be stuck. Our guide on the engine management light explains what the different light patterns mean and when it is safe to keep driving.

What Causes a P20EE Fault Code?

The code tells you the system is not cleaning the exhaust well enough. It does not tell you why. These are the usual causes, starting with the most common:

• Poor quality or old AdBlue. AdBlue goes off over time, especially in hot weather. Cheap or watered-down AdBlue may not be strong enough to do the job.
• A faulty NOx sensor. If one of the sensors gives a wrong reading, the computer may think the system is failing when it is fine. This is one of the most common causes. Our guide on the faulty NOx sensor code P13DF explains how these sensors fail.
• A blocked or crystallised AdBlue injector. AdBlue can dry into hard white crystals around the injector tip. This stops the right amount being sprayed into the exhaust.
• A weak AdBlue pump or heater. If the pump cannot push enough fluid, or the heater fails in cold weather, the system cannot dose properly.
• An exhaust leak. A leak before the sensors lets outside air in and throws off the readings.
• An engine making too much NOx. A sticking EGR valve or a faulty injector can make the engine produce more NOx than the SCR system can deal with.
• Out-of-date engine software. Some makers have released software updates to fix false P20EE codes.
• A worn or damaged SCR catalyst. This is the least common cause, but the catalyst can wear out, crack or get coated in deposits over time.

Is It Safe to Drive With a P20EE Code?

For a short distance, usually yes. The car is not likely to break down on the spot, and it is fine to drive carefully to a garage.

It is not a code to ignore, though, for three reasons:

1. The no-start countdown. If your car shows a countdown, the clock is ticking. Once it hits zero, the engine may not start again until the fault is fixed.
2. Extra pollution. Your car will be pumping out far more harmful gas than it should.
3. An MOT fail. An engine warning light that shows a fault is a major defect at the MOT. The government’s MOT inspection manual also says testers must check that emissions parts, including selective catalytic reduction parts, are fitted and not tampered with.

How We Find the Real Cause

Because so many different faults can set P20EE, guessing is a costly way to fix it. Swapping parts until the light goes out can mean paying for things you never needed. A proper check includes:

1. Reading every stored code, not just P20EE. Codes for NOx sensors, the AdBlue pump or the injector often point straight to the real problem.
2. Testing the AdBlue quality to make sure it is strong enough and not old or watered down.
3. Checking live data from both NOx sensors while the engine runs to see if they are giving sensible readings.
4. Running a dosing test to check that the pump and injector are spraying the right amount of AdBlue.
5. Looking for exhaust leaks and checking the injector for crystal build-up.
6. Checking for software updates from the car’s maker.
7. Road testing the car after any repair to make sure the code does not come back.

Our car diagnostics in East Ham use up-to-date equipment to carry out all of these checks, so you only pay to fix what is actually wrong.

How to Fix a P20EE Fault Code (and Typical Costs)

The right fix depends on what the check finds. As a rough guide:

These figures vary by make and model, and we will always give you a clear price before any work starts.

One thing we will never do is remove or switch off the AdBlue system. Doing so is against the law, it causes an MOT fail, and it can make your insurance invalid.

How to Stop P20EE Coming Back

• Use good quality AdBlue. Look for the ISO 22241 mark on the bottle, and never top up with water.
• Do not let AdBlue sit for months. Buy it as you need it, and store it out of direct sunlight.
• Keep the AdBlue tank topped up. Running it very low can let air into the system and damage the pump.
• Never put AdBlue in the diesel tank. It can cause serious damage to the fuel system. If it happens, do not start the engine.
• Take the car for a longer drive now and then. Around 20 to 30 minutes at a steady speed helps the whole exhaust system work as it should.
• Keep up with servicing. A healthy engine makes less NOx in the first place, which takes the strain off the SCR system.

Why Choose First Auto Centre

We are an RAC-approved garage on Lawrence Road in East Ham, and our mechanics have over 20 years of experience between them. We work on all makes, with extra know-how on Audi, BMW and Mercedes-Benz, many of which use AdBlue diesel engines. We find the real reason behind a P20EE code, explain it in plain English, and price the job clearly before we pick up a spanner.

We are open Monday to Saturday, 09:00 to 18:00. Call us on 020 4539 0131 or drop in at 1 Lawrence Road, London E6 1JN.

Quick Questions

Will topping up AdBlue clear the P20EE code?

Sometimes, if the old AdBlue was weak or had gone off. You may need to drain the tank first, as fresh AdBlue mixed with bad AdBlue may still not be strong enough. If the light comes back, there is another fault to find.

Can I just clear the code with a code reader?

You can, but it will not fix the cause. The code will usually come back within a few journeys, and a no-start countdown may carry on running in the background.

Does P20EE mean I need a new catalyst?

Not usually. Most cars with this code are fixed with fresh AdBlue, a new sensor, an injector clean or a software update. A new catalyst is only needed when the old one is truly worn out.

Will P20EE cause an MOT fail?

Yes, it can. An engine warning light showing a fault is a major defect, so it is best to get the code sorted before your test.

How long does it take to fix?

Testing and a simple fix, such as an AdBlue refill or software update, can often be done the same day. A new sensor, pump or catalyst may take longer if parts need ordering.

Book Your P20EE Check in East Ham

A P20EE code will not go away on its own, and if your car shows a no-start countdown, waiting could leave you stranded. Call us on 020 4539 0131 or pop in to 1 Lawrence Road, London E6 1JN, and we will find out exactly what is going on and give you a clear price before any work begins.