BMW fault 4C0E - Exhaust gas temperature sensor (pre-SCR)
P242A · Plausibility, measured vs. calculated too high
4C0E indicates a plausibility error on the exhaust gas temperature sensor before the SCR catalyst on AdBlue-equipped BMW diesels. The DDE uses this reading to regulate urea injection, activating dosing only when exhaust temperatures support efficient NOx conversion in the SCR catalyst.
This code sets when the measured pre-SCR temperature diverges from the DDE's calculated value beyond the allowed threshold for more than 60 seconds. The check only runs when no electrical faults are stored for the sensor.
- Module
- DDE
- Severity
- Warning
- Likely cause
- Drifted exhaust gas temperature sensor
- Common fix
- Replacing the pre-SCR exhaust gas temperature sensor
Symptoms
- MIL (check engine light) illuminated.
- Reduced or disabled urea injection, triggering secondary SCR and NOx sensor faults.
- AdBlue warning or power-reduction countdown if the SCR system stays inactive for extended driving.
Common causes
- Drifted temperature sensor from sustained heat exposure downstream of the DPF, where regeneration events can push temperatures past 600°C.
- Corroded wiring or connector pins from road salt and moisture along the undercarriage harness routing.
- Urea crystallization on the sensor tip from dosing at below-optimal temperatures during a prior fault condition.
- Exhaust leak at the DPF-to-SCR connecting pipe introducing cooler air around the sensor.
Diagnosis
- Read all stored codes. Check for electrical faults on this sensor and for companion SCR system faults (NOx sensor, urea injector, SCR efficiency). Multiple SCR faults suggest a systemic issue.
- With the engine cold, read the live sensor value in ISTA or INPA. It should match ambient temperature closely. A large deviation with the engine off confirms a sensor or wiring problem.
- With the engine warmed up, compare the pre-SCR reading against the pre-DPF sensor if equipped. The pre-SCR value should be lower due to heat loss across the DPF, but both should track load changes in parallel.
- Inspect wiring from the sensor connector back toward the engine harness for heat damage, chafed insulation, or corroded pins, particularly near exhaust hangers and heat shields.
- Check the sensor tip for white urea crystal deposits and inspect nearby exhaust joints for soot staining that indicates leaks.
Resolution
Replace the pre-SCR exhaust gas temperature sensor. Clear all codes after installation and drive at sustained highway speed to confirm urea injection resumes and the code does not return.