AdBlue Fault Codes Explained: P20EE, P204F, P2002 & More (UK Guide)
I Fix AdBlue | November 2025
Introduction
Modern diesel vans and cars use AdBlue (SCR) systems to cut NOx emissions. When something goes wrong, the ECU logs fault codes and often puts the vehicle into limp mode or even starts a “no start” countdown.
If you’ve just scanned your vehicle and seen codes like P20EE, P204F or P2002, this guide will help you understand:
- What each common AdBlue‑related code actually means
- Typical causes in the real world
- What usually fixes it
- Where to read more about repairs, deletes and UK‑typical costs
For a step‑by‑step reset process after repairs, you can also see our AdBlue fault codes reset guide.
How AdBlue Fault Codes Work
The ECU constantly monitors sensors around the AdBlue and exhaust system – NOx sensors, temperature sensors, pressure sensors, AdBlue level and pump performance. When readings go outside the expected range, it stores a diagnostic trouble code (DTC).
Some codes are “soft” and only light the engine warning lamp. Others are “hard” and will:
- Trigger limp mode
- Start a countdown such as “0 miles until no start”
- Prevent the engine from starting once the countdown hits zero
If you’re already in a countdown situation, this article pairs well with how to clear a no‑start countdown without the dealer .
Key AdBlue‑Related Fault Codes
P20EE – SCR NOx Catalyst Efficiency Below Threshold
What it means: The ECU thinks the SCR catalyst isn’t reducing NOx emissions effectively.
Typical causes:
- Failing or contaminated SCR catalyst
- Faulty NOx sensor giving incorrect readings
- AdBlue dosing problems (pump or injector)
- Long‑term driving with other AdBlue faults present
What usually fixes it:
- Check and, if needed, replace the NOx sensor
- Test AdBlue pump and injector performance
- In severe cases, replace the SCR catalyst
For help deciding whether to replace the NOx sensor or look at delete options, see NOx sensor delete vs replacement and signs your NOx sensor is failing .
P204F – Reductant System Efficiency Below Threshold
What it means: The AdBlue (reductant) system as a whole isn’t performing as expected.
Typical causes:
- Low‑quality or contaminated AdBlue
- AdBlue pump not delivering enough pressure
- Blocked or sticking AdBlue injector
- Frozen or crystallised AdBlue in pipes
- SCR catalyst starting to fail
What usually fixes it:
- Drain and refill with fresh, good‑quality AdBlue
- Test and, if needed, replace the pump or injector
- Flush the system if crystals or contamination are present
We cover real‑world fixes for repeated AdBlue failures in why your AdBlue system keeps failing again and again .
P2002 – DPF / NOx Trap Efficiency Below Threshold
What it means: The DPF or NOx trap isn’t cleaning exhaust gases properly.
Typical causes:
- Blocked or overloaded DPF
- Short‑journey driving preventing regeneration
- Sensor faults around the DPF or exhaust
- Issues upstream in the AdBlue or EGR systems
What usually fixes it:
- Proper DPF clean or regeneration
- Repairing any AdBlue or EGR faults that caused the overload
- In some cases, DPF replacement or a DPF delete solution
If you’re weighing up long‑term options on a high‑mileage work van, our AdBlue delete vs repair guide and the DPF delete service page are useful next reads.
P207F / P20BA – Reductant Quality / Level Issues
What they mean: The ECU has detected a problem with AdBlue quality, concentration or level.
Typical causes:
- AdBlue mixed with water, diesel or dirt
- Very old AdBlue that has degraded
- Level sensor mis‑reading the tank
What usually fixes it:
- Drain the tank and refill with fresh, branded AdBlue
- Clean or replace the level/quality sensor if readings stay wrong
For a deeper look at contamination and real‑world fixes, see urea delete and real‑world fixes .
P2200–P2202 – NOx Sensor Circuit Faults
What they mean: The ECU can’t communicate properly with a NOx sensor.
Typical causes:
- Failed NOx sensor
- Damaged wiring or corroded connector
- Water ingress in the loom
What usually fixes it:
- Check wiring and connectors for damage
- Replace the affected NOx sensor
- Clear codes and road‑test to confirm
Because NOx sensors are expensive, many owners compare replacement with NOx sensor delete options .
P2BAE / P2BAF – SCR NOx Exceeded Limit
What they mean: The vehicle has exceeded allowed NOx emissions despite AdBlue dosing.
Typical causes:
- Combination of NOx sensor and SCR efficiency issues
- AdBlue dosing problems plus high‑load driving
- Long‑term running with other unresolved AdBlue faults
What usually fixes it:
- Address any underlying P20EE/P204F/P2002 codes first
- Verify correct AdBlue dosing and SCR operation
What Happens If You Ignore AdBlue Fault Codes?
Ignoring AdBlue‑related codes can lead to:
- Repeated limp mode events
- “0 miles until no start” countdowns
- Permanent no‑start until the system is repaired and reset
- Long‑term damage to the SCR catalyst or DPF
If you’re already seeing a no‑start countdown, read 0 miles until no start – here’s how to get your van moving again for immediate next steps.
Typical UK Cost Ranges (Guide Only)
These figures are general UK market ranges to help you understand the scale of repairs. They are not specific prices for any one garage or provider. For a full breakdown by fault type, see our complete AdBlue repair cost guide .
- Diagnostic scan: around £50–£120
- AdBlue drain and refill: around £150–£300
- NOx sensor replacement: often £300–£550
- AdBlue pump replacement: often £400–£700
- SCR catalyst replacement: often £600–£1,200
Repair vs Delete – Which Route Makes Sense?
On newer, low‑mileage vehicles, repair is usually the obvious choice. On older, high‑mileage work vans, repeated AdBlue and NOx‑related codes can become an expensive cycle of parts and labour.
Many owners compare long‑term repair costs with:
- AdBlue delete vs repair – what’s right for your van?
- Why some drivers choose an AdBlue delete
- Can you still pass an MOT after emissions deletes?
Before deciding, it’s important to understand the legal and MOT position. Our article AdBlue delete legal UK explains the current situation in detail.
Related Guides
- AdBlue fault codes reset – step‑by‑step
- Why your AdBlue system keeps failing again and again
- What causes AdBlue faults and how to fix them
- Why some drivers choose an AdBlue delete and what you should know
- AdBlue problems – service overview
Need Help With an AdBlue Fault Code?
If your scan tool is showing one or more of these codes and you’re not sure what to do next, the safest option is a proper diagnostic session with someone who specialises in AdBlue, NOx and SCR systems.
We offer mobile diagnostics and AdBlue solutions across Leicester and the surrounding areas. You can get in touch via the contact us page, or browse our AdBlue delete and AdBlue problems service pages for more information.
