BMW fault 4EED - Speed sensor (output)
P0721 · Range/performance
4EED is stored by the EGS when the output speed sensor (NOUT) signal is present but falls outside the expected range or behaves erratically during driving. The sensor measures output shaft speed for gear ratio calculation and shift control. Unlike a complete circuit failure, the signal here exists but does not match what the EGS expects.
The EGS receives a signal from the output speed sensor but it does not correlate with the expected output shaft speed based on other inputs. This check runs only when the vehicle is moving at 10 km/h (6 mph) or faster, as the EGS needs actual shaft rotation to evaluate signal quality. The MIL illuminates on the second drive cycle.
- Module
- EGS
- Severity
- Warning
- Likely cause
- Degrading output speed sensor or intermittent wiring connection
- Common fix
- Replacing the output speed sensor
Symptoms
- MIL (check engine light) illuminated on the second drive cycle.
- Intermittent harsh or erratic gear shifts, particularly at higher speeds.
- Transmission may enter failsafe mode, often after extended driving or at highway speed.
Common causes
- Degrading output speed sensor producing a weak or noisy signal that drifts out of the expected range.
- Intermittent wiring connection that passes a static continuity check but fails under vibration or heat.
- Metallic debris from transmission fluid contamination accumulating on the sensor tip and distorting the signal.
- Loose or corroded connector pins with high resistance under load.
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.
- If the fault is active, perform a continuity check on the external transmission wiring harness. Even though this is a signal quality fault, intermittent wiring problems are a common root cause. Wiggle the connectors and harness while monitoring for signal dropout.
- Check the output speed sensor signal. If the RPM signal is not within the specified range or drops out during driving, the sensor (NOUT) has failed or is contaminated.
- If the sensor and wiring both check out, the EGS control module may be misinterpreting the signal. This is uncommon.
Resolution
Repair or replace the external wiring harness if an intermittent connection or damage is found. If the sensor signal is out of range or erratic with good wiring, replace the output speed sensor (NOUT). Perform a transmission fluid and filter service during the repair if the fluid is contaminated with metallic debris, as contamination on the sensor tip can cause signal quality faults. As a last resort, replace the EGS control module and reset adaptation data. Clear the fault memory and road test above 10 km/h to confirm the code does not return.