P204F–P205F Reductant System Performance Bank 1

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

P204F: Reductant System Performance Bank 1.

P204F is a manufacturer-specific powertrain code (the engine or gearbox). Its meaning is set by the carmaker rather than by the SAE standard, so a generic OBD-II reader will often show it as an unknown code or misread it.

What to check first

Cheapest and fastest checks first:

1. Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
2. Check the connector at the part for corrosion, and that it is fully seated.
3. Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

This is the car reporting that its AdBlue system - the SCR system - is not cleaning up nitrogen oxides as well as it should. The engine control unit judges the result from the NOx sensor downstream of the SCR catalyst, comparing it against how much AdBlue it metered in; on many VAG diesels there is a second NOx sensor upstream as well, and on others the upstream value is modelled rather than measured. When the reduction achieved falls short of what that dose should have produced, this code is stored. It is an outcome code, not a component code: it says the result was wrong, not what broke. "Bank 1" is largely nominal on the four-cylinder TDIs that make up most of the VAG range, since they have one exhaust and one SCR system; on a V6 it identifies one side of the engine. On VAG diesels with SCR an emissions fault of this kind normally lights a warning and starts a countdown - presented as remaining range on some models and as a number of remaining starts on others - after which the engine will not start again. The car does not cut out while you are driving, but once you switch it off it will not go again, so this is not one to clear and ignore.

Most likely causes, in order

1. Poor, old or contaminated AdBlue. AdBlue is a precise mix - 32.5 per cent urea in demineralised water, to the ISO 22241 specification - and it has a shelf life, degrades in prolonged heat, and is ruined outright by anything else going into the tank, whether that is water, screenwash or diesel from a mistaken top-up. It is the cheapest thing on this list to eliminate and it costs nothing to ask whether anyone has topped it up from a container that has been sitting open in a shed. A refractometer reads the concentration in seconds. Some VAG models also carry a fluid quality sensor, so check whether a separate reductant quality fault is stored alongside this one - if it is, that points straight here.
2. Crystallised AdBlue at the dosing injector. The fluid dries into hard white crystals around the injector tip and in the mixer section of the exhaust, so the spray pattern collapses or is partly blocked and less fluid reaches the catalyst than the ECU believes it has delivered. Look at the injector where it enters the exhaust and at the pipework around it - white crusty deposits are unmistakable. Lots of short cold journeys make this much more likely, because the exhaust never gets hot enough to keep the area clean.
3. A NOx sensor reading wrongly. These sit in hot exhaust gas, age, and drift slowly, and a drifted sensor can make a perfectly healthy system look like it is failing. They are a frequent failure on VAG diesels - and also one of the most needlessly replaced parts on them, because they are expensive and easy to blame. Check the sensor's connector and the wiring to its module first. Then, with the engine warm and cruising steadily, watch the NOx signals in live data - both of them where two are fitted. One stuck at a fixed number, pinned implausibly high, or simply not responding as load changes is the suspect. Replace on measured evidence, never on reputation.
4. A leak or blockage in the dosing system, or a tired SCR catalyst. A weeping line or pump seal means less fluid arrives than the ECU counted out, and a blocked filter or strainer in the AdBlue pump does the same. A catalyst that has been poisoned by years of out-of-specification fluid, or by oil or coolant passing through it, loses its ability to convert at all. This is the expensive end of the list and it belongs last: only consider it once the fluid, the injector and the sensors have all been proven good.

Worth knowing: The two cheapest things to check - the fluid itself and crystallisation at the dosing injector - account for a good share of these faults. The NOx sensor and the SCR catalyst are the expensive parts and the ones most often fitted needlessly, so make anyone proposing either show you the live data that condemns it. Be aware too that erasing the code does not clear the restart countdown on its own: after a genuine repair the car normally has to complete a proper drive cycle at temperature before the monitor passes and the warning goes out. If the countdown is already low, book the car in rather than driving on it - when it reaches zero the engine will not restart, and that happens wherever you happen to have parked.

Common questions

What does P204F mean?

P204F means Reductant System Performance Bank 1. The part responds, but not the way the ECU expects under these conditions - a wear or plausibility problem rather than a dead circuit.

Can I drive with P204F?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

Related reading

• 2.0 TDI EA288 problems: the owner's and buyer's guide
• DPF regeneration on a VAG diesel: what it is and what blocks it
• Why the same fault code has completely different causes