Fault code
BMW fault 4C09 - Exhaust gas temperature sensor (pre-DPF)
P042A · Plausibility, measured vs. calculated too high
4C09 flags a plausibility error on the exhaust gas temperature sensor upstream of the diesel particulate filter. The DDE uses this sensor to manage DPF regeneration timing and protect downstream components from excessive heat.
The DDE compares the measured pre-DPF temperature against a calculated value based on engine load and fueling. This code sets when the difference exceeds the plausibility window for more than 60 seconds. The check only runs when no electrical faults exist for the sensor.
- Module
- DDE
- Severity
- Warning
- Likely cause
- Drifted exhaust gas temperature sensor
- Common fix
- Replacing the pre-DPF exhaust gas temperature sensor
Symptoms
- Check engine light illuminated.
- DPF regeneration cycles delayed or aborted, causing progressive soot buildup and secondary DPF faults.
- Increased fuel consumption from more frequent or prolonged regeneration attempts as the DDE compensates.
Common causes
- Drifted thermistor element from prolonged heat cycling, producing readings that diverge from the DDE's calculated model.
- Damaged or corroded sensor wiring along the exhaust system. The harness is exposed to extreme heat and road spray.
- Exhaust leak between the turbocharger and DPF introducing ambient air that cools the sensor locally.
- Soot buildup on the sensor tip insulating it from the exhaust gas stream.
Diagnosis
- Read all stored codes. Confirm no electrical faults (open circuit, short to ground, short to B+) exist for this sensor. The plausibility check is disabled while electrical faults are active.
- With the engine off and cold, read the live sensor value in ISTA or INPA. It should closely match ambient temperature. A large deviation points to sensor or wiring failure.
- Start the engine and monitor the reading as it warms. Temperature should rise smoothly with engine load. Erratic jumps or a flatlined reading indicate sensor failure.
- Inspect the wiring harness from the sensor connector toward the DDE for heat damage, melted insulation, or corroded pins.
- Check the exhaust system between the turbocharger and DPF for leaks by looking for soot traces at joints and gasket surfaces.
Resolution
Replace the pre-DPF exhaust gas temperature sensor, clear codes, and drive at highway speeds to confirm the plausibility check passes.
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Last updated Jun 17, 2026 · Suggest an edit