Fault code P20EE00 means the SCR catalyst isn’t reducing NOx as expected. It’s one of the most common AdBlue-related errors across vans, especially Mercedes, Ford, and VW models. Here’s what it means, how we diagnose it, and how to clear it for good.
What the P20EE00 code means
P20EE00 translates to “SCR NOx Catalyst Efficiency Below Threshold.” The ECU is detecting higher nitrogen oxide (NOx) levels than allowed after the SCR system, meaning AdBlue dosing or the catalyst’s reaction isn’t working properly.
This doesn’t always mean the catalyst itself has failed — it can also be caused by inaccurate NOx sensor readings, poor dosing, or even old AdBlue fluid.
Symptoms you’ll notice
- AdBlue or engine warning light on the dash.
- Stored P20EE00 and possibly related codes like P229F or P204F.
- Reduced power or “emissions fault” message.
- AdBlue consumption dropping to zero (system shutting off dosing).
- Countdown warnings like “0 miles until no start.”
Main causes of the P20EE00 fault
- NOx sensor malfunction: Incorrect readings make the ECU think the catalyst isn’t working.
- AdBlue quality issues: Contaminated or aged fluid reduces dosing effectiveness.
- Blocked or failed injector: No AdBlue reaching the catalyst equals poor conversion efficiency.
- Exhaust leaks before the catalyst: Skews sensor readings and oxygen levels.
- SCR catalyst ageing: Long-term exposure to heat and contamination weakens efficiency.
Our on-site diagnostic process
We diagnose P20EE00 faults directly on your driveway or at your workplace — no tow or dealer visit needed. Here’s how we pinpoint the cause:
- Read full code set: We check for associated codes like P20E8 (low pressure) or P204F (system performance).
- Live NOx readings: We compare upstream and downstream sensor data to confirm catalyst efficiency.
- AdBlue dosing test: Using diagnostic software, we confirm proper injector pulse and pressure.
- Visual and leak checks: We inspect for cracked lines or injector crystallisation.
- Fluid test: We verify AdBlue quality using refractometer or test strips.
How we fix the fault
- Sensor replacement or recalibration: If readings are inconsistent, we fit or code new sensors.
- SCR catalyst cleaning or replacement: If efficiency is genuinely low, we restore flow or replace the unit.
- AdBlue line and injector service: Removes blockage and restores correct dosing.
- Software reset: After repairs, we reset the adaptation and catalyst efficiency monitor.
Linked faults and guides
- Signs your NOx sensor is failing
- NOx sensor delete vs replacement
- Why AdBlue systems keep failing
- How to reset AdBlue fault codes
When delete becomes an option
In rare cases where SCR components keep failing or costs outweigh benefits, some fleet owners choose to disable the AdBlue system in software. Learn the pros and cons in AdBlue delete vs repair – what’s right for your van.
Preventing P20EE00 in the future
- Always use certified AdBlue (ISO 22241).
- Don’t let the system run empty — air in the lines can affect dosing accuracy.
- Keep the cap sealed and refill from sealed containers only.
- Run periodic diagnostics even when no warning is on — early detection saves money.
Book a mobile SCR system diagnostic
We fix P20EE00 and related AdBlue faults on-site across Leicestershire and nearby. See our SCR system repair service or book a visit today.
Helpful links
Need your P20EE00 warning gone before MOT?
Call 07349 821 999 or email info@ifixadblue.co.uk. Fast mobile diagnostics and repair across Leicestershire.
