P24ED
NH3 Sensor Calibration Memory
The NH3 slip sensor stores its individual calibration coefficients in non-volatile memory (EEPROM/flash). The ECM cannot read valid calibration values — either checksum fails, the memory is empty after replacement, or communication with the sensor electronics is broken. Without the trim values the raw ammonia reading is unreliable.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- New sensor installed but not coded/learned to the ECMCommon
- Corrupted calibration memory after voltage dropPossible
- Module software outdated, reflash may resolveLess likely
Parts involved
- Nh3 Sensor
- Sensor Eeprom
P24ED 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.