P21CE
Reductant Quality Module Performance
DEF quality module reports an internal performance fault — self-test failed or measurement loop unable to converge. The module may need replacement.
- 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.
- Internal module faultCommon
- Module software outdated, reflash may resolveLess likely
- Supply voltage outside specificationLess likely
Parts involved
- Reductant Quality Sensor
P21CE 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.