PACCAR MX (2004 Emissions) Fault Code: U1851 NH3 Sensor Software Verification – Incorrect

SET CONDITIONS:
The PCI ECU detects that no or an incorrect calibration verification number (CVN) is received from the NH3 sensor.

DIAGNOSTIC CONDITION:
This diagnostic runs once when the ignition is keyed on.

RESET CONDITION:
This DTC changes to inactive next time the ignition is keyed on and the fault is no longer detected.

POSSIBLE CAUSES:
1. No or intermittent CAN communication with the NH3 sensor
2. Incorrect type of NH3 sensor installed
3. Faulty NH3 sensor

ADDITIONAL INFORMATION:
When the ignition is keyed on, the PCI ECU (D420) queries the NH3 sensor (F869) for a calibration verification number (CVN).

CVN is the calibration verification number and is the result of a check performed on the software values stored in emission-related ECUs (OBD legislation).