BMW fault 4EEC - Speed sensor (output)
P0720 · Circuit fault
4EEC is stored by the EGS when it detects an electrical circuit fault on the output speed sensor (NOUT). This sensor measures the rotational speed of the transmission output shaft. The EGS uses this signal to calculate gear ratios, control shift timing, and regulate line pressure.
The EGS has detected that the output speed sensor signal path is compromised: the signal is absent, interrupted, or electrically invalid. The monitoring runs with the engine running and battery voltage between 9V and 16V. The MIL illuminates on the second drive cycle.
- Module
- EGS
- Severity
- Warning
- Likely cause
- Damaged external transmission wiring harness
- Common fix
- Repairing the wiring or replacing the output speed sensor
Symptoms
- MIL (check engine light) illuminated on the second drive cycle.
- Transmission enters failsafe mode.
- Harsh, delayed, or absent gear shifts.
Common causes
- Damaged, chafed, or broken wiring in the external transmission harness, particularly near heat sources or sharp edges.
- Loose or corroded connector pins at the EGS control module or transmission case connector.
- Failed output speed sensor with an open or shorted coil.
Diagnosis
- Read the fault memory and note whether the fault is active or stored. If passive, check plug connections at the EGS and transmission for correct detent engagement and loose contacts. Inspect the external transmission harness for damaged insulation, abrasion, crimping, or crushing. Perform a continuity check on the harness.
- If the fault is active, perform a continuity check on the external transmission wiring harness directly. A failed continuity check confirms a wiring break.
- If the harness checks out, check the output speed sensor signal. If the RPM signal is absent or not within the specified range, the sensor (NOUT) has failed.
- If the sensor and wiring are both confirmed good, the EGS control module may be at fault.
Resolution
Repair or replace the external transmission wiring harness if damage or a continuity failure is found. If the sensor signal is absent or out of range with good wiring, replace the output speed sensor (NOUT). As a last resort, replace the EGS control module and reset the adaptation data after replacement. Clear the fault memory and road test to confirm the code does not return.