P204F
Reductant System Performance
The SCR (selective catalytic reduction) system using AdBlue/DEF is not delivering the expected NOx conversion. Causes range from poor reductant quality, dosing module faults, heater failure on the AdBlue tank in cold weather, or a degraded SCR catalyst.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Common causes
Ordered by how often each turns out to be the cause.
- Poor reductant quality or contaminationCommon
- Dosing module clogged with urea crystalsPossible
- SCR catalyst aged or contaminatedPossible
Parts involved
- SCR Catalyst
- Adblue Dosing Module
- Adblue Tank Heater
- NOX Sensor
P204F is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.