PACCAR MX (2004 Emissions) Fault Code: P0666 Internal ECU Temperature – Data Erratic, Intermittent or Incorrect, at Ignition On

PACCAR MX (2004 Emissions) Fault Code: P0666 Internal ECU Temperature – Data Erratic, Intermittent or Incorrect, at Ignition On

SET CONDITIONS:
The PCI ECU (D420) detects that the measured internal ECU temperature differs more than 3?C [5.4?F] from the average of other temperature sensors on the engine for more than 5 seconds (after the ignition has been keyed off for at least 8 hours).

DIAGNOSTIC CONDITION:
This diagnostic runs:
1. When the ignition has been keyed off continuously for at least 8 hours
2. Difference between the coolant temperature and ambient temperature is less than 15?C [27?F]
3. Ambient temperature is more than -20?C [-4?F]

CORRESPONDING OBD READINESS MONITOR:
None

RESET CONDITION:
The 8-hour ignition off diagnostic consists of three separate steps:
1. The vehicle ignition may NOT be keyed on or engine started for 8 to 10 consecutive hours (ideal situation would be overnight).

2. After the 8-10 consecutive hours, key on the ignition (NO engine start) and wait for 10 seconds to allow the system to power-up and the diagnostics to run.

3. Start the engine and let it idle for 2 minutes. This DTC changes to inactive when 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:
Faulty temperature sensor, internally in ECU. If the fault recurs, replace the ECU.

ADDITIONAL INFORMATION:
1. The PCI ECU temperature is monitored by an internal temperature sensor in the printed circuit board. This temperature sensor is not serviceable.

2. For this diagnostic, the internal PCI ECU temperature sensor reading is compared with an average of other temperature sensors on the engine after the ignition has been keyed off for at least 8 hours.