P20EE: SCR NOx catalyst efficiency below threshold (bank 1)
The SCR catalyst, which uses AdBlue to clean nitrogen oxides (NOx) from a diesel’s exhaust, isn’t reducing them enough, judged by the NOx sensors.
Updated
What P20EE means
Many modern diesels clean nitrogen oxides (NOx) from the exhaust with an SCR catalyst. AdBlue, a solution of urea in water, is sprayed into the exhaust in front of it, and the catalyst turns the NOx into nitrogen and water.
NOx sensors in front of and behind the catalyst tell the engine computer how well it works. When too little NOx is removed, it stores P20EE.
P203F, P2047, P20B9 and other AdBlue system codes
These codes concern the AdBlue system itself: the level (P203F), the injector that sprays AdBlue into the exhaust (P2047), the heaters that thaw it and keep it liquid in frost (P20B9, P202A), its pressure (P20E8), its quality (P207F) and the system as a whole (P204F). Each has its own meaning and causes below.
Under EU rules, a low AdBlue level, the wrong quality or interrupted dosing all lead to the same warning and countdown as P20EE. For P203F, topping up the AdBlue is often all that’s needed, though some cars need a reset after running empty.
P203F: Reductant Level Too Low
The AdBlue tank is nearly empty, or its level sensor says so.
Common causes: adblue tank nearly empty, faulty or crystallised level sensor, wiring or connector fault.
Driving: Top up the AdBlue soon. When it runs low, the dashboard counts down a distance: once it reaches zero, the engine won’t restart until AdBlue is added, and some cars also need a reset. If the tank is full, have the level sensor checked.
P2047: Reductant Injection Valve Circuit/Open (Bank 1 Unit 1)
The engine computer has found a fault in the circuit of the AdBlue injector (dosing valve), so AdBlue may not be sprayed into the exhaust.
Common causes: faulty adblue injector, wiring or connector fault, fault in the adblue (scr) control module.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
P20B9: Reductant Heater A Control Circuit/Open
The circuit of a heater in the AdBlue system has a fault. The heaters keep AdBlue liquid in cold weather, so in frost the system may not be able to dose it.
Common causes: faulty heater, often built into the adblue tank unit, wiring or connector fault.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
P202A: Reductant Tank Heater Control Circuit/Open
The circuit of a heater in the AdBlue system has a fault. The heaters keep AdBlue liquid in cold weather, so in frost the system may not be able to dose it.
Common causes: faulty heater, often built into the adblue tank unit, wiring or connector fault.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
P20E8: Reductant Pressure Too Low
The pressure in the AdBlue dosing system is too low.
Common causes: low adblue level, faulty adblue pump, leaking or blocked adblue line, fault in the adblue (scr) control module.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
P207F: Reductant Quality Performance
The fluid in the AdBlue tank doesn’t seem to have the right quality, judged by a quality sensor or by how well the NOx is cleaned.
Common causes: diluted, contaminated or wrong fluid in the adblue tank, old adblue, faulty adblue quality sensor.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
P204F: Reductant System Performance
The AdBlue (SCR) system as a whole isn’t performing as expected, for example its pressure or the NOx cleaning misses the target. Other AdBlue codes stored with it usually point to the cause.
Common causes: low, wrong or contaminated adblue, leaking adblue line, worn adblue pump, clogged adblue injector.
Driving: Probably OK to drive short-term, but have it checked soon. Diesels with AdBlue show a warning with a distance countdown for faults like this: once it reaches zero, the engine may not restart, or the speed may be limited, until it’s repaired.
Can I keep driving?
Check soon
Probably OK to drive short-term, but have it checked soon: emissions are higher. If the dashboard shows an AdBlue or emissions warning with a distance countdown, the engine may not restart, or the speed may be limited, once it reaches zero.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Diesels with AdBlue warn the driver when the system has a fault, and then count down a distance. At the end, depending on the car, the engine won’t restart or the speed is limited until it’s repaired. Don’t wait for the countdown.
Symptoms
- The check engine light, often with an AdBlue or emissions warning.
- On some cars, a distance countdown on the dashboard.
- White deposits around the AdBlue injector on the exhaust, on some cars.
Common causes
- Poor-quality, diluted or contaminated AdBlue. AdBlue degrades with heat and age, and anything mixed in, such as water or diesel, spoils it. Fill only from a sealed container or a pump.
- Faulty NOx sensor. A sensor that misreads can make a working system look poor.
- Clogged AdBlue injector or a dosing fault. AdBlue can crystallise and block the injector, so too little reaches the catalyst.
- Aged or damaged SCR catalyst. Usually the last thing to suspect, after the AdBlue, the sensors and the dosing.
Typical fixes
- Have the AdBlue quality checked. If it’s diluted or contaminated, have the tank drained and the system flushed; don’t just top it up. If diesel got in, don’t drive: see the questions below.
- Have the AdBlue injector checked for crystals, and the dosing system tested.
- Have the NOx sensors checked.
- Replace the SCR catalyst only when the other causes are ruled out.
Questions
Can I drive with P20EE?
Probably for a short time, but have it checked soon: emissions are higher. If the dashboard shows an AdBlue or emissions warning with a distance countdown, the engine may not restart, or the speed may be limited, once it reaches zero.
Can bad AdBlue cause P20EE?
Yes. AdBlue that is old, diluted or contaminated cleans less, and it’s one of the first things to check. If diesel or anything else got into the AdBlue tank, don’t start the engine, or switch it off as soon as it’s safe. Have the car taken to a workshop: the tank and the AdBlue system need draining and flushing, and parts such as the pump or the injector may need replacing.