420641
High420641 — Park Lock: Engagement Fault
Description
What the Code Means
Code 420641 points to a fault in the park lock or the park position sensor in the automatic transmission. It shows up on BMW F-series cars, especially on a cold engine with low ATF level or after mechatronics work where the sensor was installed incorrectly. The transmission control module (TCM) sees a mismatch between the commanded and actual park lock position, which prevents the engine from starting after a transmission replacement.
Symptoms
Symptoms
- Engine won't start
- Transmission locked in P (Park)
- Warning message on the display: “Parking lock defective”
- Intermittent fault on cold mornings
Causes and fixes
Diagnosis and Repair
Step 1: Check the ATF Level
- Check the transmission fluid level and top off if needed (use ZF Lifeguard 8).
Step 2: Check the Park Position Sensor
- Inspect the alignment and seating of the park lock position sensor.
Step 3: Adapt the Park Position
- Relearn the park lock position with ISTA or another diagnostic tool.
Step 4: Replace the Mechatronics Unit
- If the sensor is faulty, replace the mechatronics unit.
Possible secondary damage
Secondary Damage
Ignoring the fault can lead to park lock failure, which creates a risk of the car rolling on its own. Continuing to drive with a low ATF level can damage the transmission.