Fault codeP204F

MODERATE

Reductant System Performance (Bank 1)

What this code means

Reductant System Performance (Bank 1).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

P204F is a generic code: it means the same thing on every make. P204F belongs to family P20xx — Fuel, air and emission controls. Its page explains how to read a code and groups all its codes by part.

Type of fault

The signal stays within its electrical limits, but its value is not plausible: it does not match what the control unit expects given the other sensors. The sensor may be drifting with age, but it may also be faithfully measuring a real mechanical problem — a leak, a blockage, an air leak. Replacing the sensor without checking does not always clear the fault.

AdBlue / SCR system

On recent diesels, the SCR system injects AdBlue, a urea solution, into the exhaust upstream of a dedicated catalytic converter, which turns nitrogen oxides into nitrogen and water. A pump, a heated tank, a level sensor and an injector make up the system, and NOx sensors check its efficiency.

What you notice

An AdBlue warning with a countdown of the kilometres left: once it reaches zero, the engine may refuse to restart. Some models also reduce power.

How to check it

Check the AdBlue level and quality first: old, diluted or contaminated fluid (water, diesel) is a frequent cause. AdBlue crystallises, so look for white deposits around the injector and the pipes. Then the tank heater and pump, and the NOx sensors. Always fill up with AdBlue that meets the ISO 22241 standard.

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
2standardised code
0Fuel, air and emission controls
4FP204F