PACCAR MX (2004 Emissions) Fault Code: U1855 Catalytic Convertor Temperature Sensors ECU Software Verification – Incorrect

PACCAR MX (2004 Emissions) Fault Code: U1855 Catalytic Convertor Temperature Sensors ECU Software Verification – Incorrect

SET CONDITIONS:
The PCI ECU detects that no or an incorrect calibration verification number (CVN) is received from the catalytic convertor temperature sensors ECU.

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

CORRESPONDING OBD READINESS MONITOR:
None

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

REPAIR VERIFICATION:
1. Clear all DTCs.
2. Test drive or operate the vehicle to meet the diagnostic conditions and set conditions.
3. Did the DTC come back? (Can be pending, inactive or active)
– No, repair successful
– Yes, repair unsuccessful

POSSIBLE CAUSES:
1. No or intermittent CAN communication with the catalytic convertor temperature sensors ECU
2. Incorrect type of catalytic convertor temperature sensors ECU installed
3. Faulty catalytic convertor temperature sensors ECU

ADDITIONAL INFORMATION:
When the ignition is keyed on, the PCI ECU (D420) queries the catalytic convertor temperature sensors ECU (D419) 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).