P20EE
PowertrainSCR NOx Catalyst Efficiency Below Threshold Bank 1
Your AdBlue / SCR system isn't converting nitrogen oxides as effectively as the ECU expects on bank 1. The selective catalytic reduction system, the part that meets modern Euro 6 and Euro 6d emissions, is underperforming.
ⓘ Information only. This page provides general educational information about fault code P20EE. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P20EE mean?
P20EE is a Powertrain (engine, transmission, fuel system) fault code. It indicates: SCR NOx Catalyst Efficiency Below Threshold Bank 1.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light
- • AdBlue warning on the dashboard, often a countdown ('engine will not restart in X miles')
- • Limp mode in some cases, particularly close to the no-restart threshold
- • Sometimes a smell of ammonia from the exhaust if dosing is uncontrolled
Possible causes
Causes commonly associated with P20EE, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. AdBlue tank empty or below operational threshold (check this first, free fix)
- 2. Failed AdBlue dosing pump or injector, the unit that sprays AdBlue into the exhaust
- 3. Failed AdBlue heater (winter problem on cars left out in cold), AdBlue freezes around -11°C
- 4. Failed NOx sensor (upstream or downstream of the SCR cat)
- 5. Wrong fluid in the AdBlue tank, drivers occasionally fill it with diesel, screenwash, or worse
- 6. SCR catalyst aged or contaminated
- 7. Software bug, some manufacturers have issued software updates specifically for P20EE
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Top up the AdBlue tank to full, this clears a meaningful percentage of P20EE codes if the tank was just low
- 2. Check live data on AdBlue dosing rate and NOx sensor readings
- 3. If it's winter and the car has been parked outside for days, the AdBlue may be frozen, the heater should clear it once the engine warms up
- 4. Inspect the AdBlue tank for contamination, if anything other than clear, slightly straw-coloured fluid, drain and refill
- 5. Check for software updates with the dealer, several manufacturers have published TSBs for this code specifically
- 6. If everything else is clean, the dosing pump or NOx sensor is the next suspect, both workshop diagnostics
Common questions about P20EE
Engine warning says I have X miles before it won't restart, what's that about? +
Modern diesels have legal emissions controls baked in. If the SCR system can't reduce NOx, the car is required by Euro 6 regulations to alert the driver and eventually refuse to restart until the issue is resolved. The countdown is real. Don't ignore it, get it diagnosed before you hit zero.
Can I keep topping up AdBlue and ignore the code? +
Sometimes that buys time but rarely fixes the underlying issue. If the dosing pump or sensor is faulty, the SCR system can't actually reduce the NOx regardless of how much AdBlue is in the tank, and the code returns. The legal restart limit will eventually catch up.
What about cars sold without AdBlue? +
Pre-2015 diesels typically don't have AdBlue (they use LNT instead, see P2000). If your car has an AdBlue filler cap, it has SCR. If it doesn't, P20EE shouldn't be possible.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →