P20EE00 Fault Code Explained – SCR NOx Catalyst Efficiency Below Threshold

If your diesel vehicle has displayed an AdBlue warning, engine management light or emissions fault, you may have had the diagnostic trouble code P20EE00 – SCR NOx Catalyst Efficiency Below Threshold (Bank 1).

This is a common fault code on modern diesel vehicles fitted with an SCR (Selective Catalytic Reduction) and AdBlue system.

But one important thing to understand is that P20EE00 does not automatically mean the SCR catalytic converter needs replacing.

The fault code is telling you that the vehicle’s computer has detected that the SCR system isn’t reducing NOx emissions as effectively as expected. It does not identify one specific failed component. Proper diagnosis is therefore extremely important before expensive parts are replaced. (BEST GROWTH — Aftertreatment Solutions)

What does P20EE00 mean?

The full description of the code is:

P20EE00 – SCR NOx Catalyst Efficiency Below Threshold (Bank 1)

The SCR system is designed to reduce harmful nitrogen oxide (NOx) emissions from a diesel engine.

The system uses AdBlue, which is injected into the exhaust system. The vehicle then monitors the NOx levels and uses this information to determine whether the SCR system is reducing emissions sufficiently.

When the ECU determines that the measured NOx reduction is below the required threshold, it can store P20EE00. (P20EE)

Does P20EE00 mean the SCR catalyst has failed?

Not necessarily.

This is one of the biggest mistakes we see with this fault code.

P20EE00 is an efficiency fault, rather than a direct statement that the catalytic converter has failed.

There are several different reasons why the vehicle may calculate that SCR efficiency is too low, including:

• Faulty NOx sensor

• Faulty AdBlue/DEF injector or dosing system

• Incorrect or contaminated AdBlue

• AdBlue dosing problems

• Exhaust leaks

• Wiring or electrical faults

• Incorrect exhaust temperatures

• Problems elsewhere in the emissions system

• A genuinely degraded or failed SCR catalyst

Manufacturer-specific diagnostic information also lists NOx sensors, AdBlue dosing, AdBlue concentration, exhaust leaks and SCR catalyst condition among the areas that should be investigated. (Ross-Tech Wiki)

Why proper diagnosis is important

Simply seeing P20EE00 on a diagnostic scanner and immediately replacing the SCR catalyst can result in a very expensive repair that may not actually fix the problem.

A proper diagnosis should establish why the vehicle believes SCR efficiency is below threshold.

Depending on the vehicle, this can involve checking live NOx sensor readings, AdBlue quality and dosing, exhaust temperatures, exhaust leaks, related fault codes and the operation of the SCR system. (BEST GROWTH — Aftertreatment Solutions)

This is why we always recommend diagnosing the cause rather than simply replacing parts based on the fault code.

Can a DPF problem be connected to P20EE00?

Potentially, yes — but this needs to be diagnosed rather than assumed.

The DPF and SCR systems are both part of the vehicle’s exhaust after-treatment system, and the conditions under which the SCR efficiency monitor operates can be affected by DPF regeneration and other exhaust-system conditions. (Ross-Tech Wiki)

For example, if a vehicle also has a significantly restricted DPF or other emissions-related faults, it may be sensible to investigate and rectify those issues before condemning an SCR component.

However, a P20EE00 fault should not automatically be blamed on the DPF. If the SCR fault remains after other known issues have been corrected, further SCR/NOx diagnosis may be required.

What symptoms can P20EE00 cause?

Depending on the vehicle and the severity of the problem, you may experience:

• Engine management light

• AdBlue or emissions warning

• SCR system warning

• Reduced engine performance

• Limp-home or reduced-power operation

• Increased emissions

• In some vehicles, additional warnings or restrictions if the problem remains unresolved

The exact symptoms vary considerably between manufacturers and models. (Ross-Tech Wiki)

What should you do if your car has P20EE00?

Don’t automatically agree to expensive parts being fitted just because the scanner says P20EE00.

The first step should be a proper diagnostic assessment.

We can investigate the vehicle’s fault codes and relevant system data to establish what is actually happening before recommending repairs.

If there is also a DPF blockage, we can assess the DPF separately and establish whether it requires cleaning. Once the known issue has been dealt with, the vehicle can be tested again to determine whether the SCR fault remains.

This approach helps prevent unnecessary parts being replaced.

Our approach

We diagnose the fault — we don’t just fit random parts and hope for the best.

P20EE00 can involve several different components, and replacing an expensive SCR catalyst without proving that the catalyst is actually at fault can be a costly mistake.

If your vehicle has P20EE00, an AdBlue warning or an SCR/NOx fault, contact us for a proper diagnostic assessment.

Advanced Remaps & DPF Cleaning

Specialists in DPF diagnostics, DPF cleaning and diesel emissions-system diagnostics.