646 Improper gear ratio in 2nd gear
How to read it
Code 646 reads as 6 flashes, pause, 4 flashes, pause, 6 flashes on the check-engine light. Jumper the self-test input to signal return at the test connector, then run the key on engine off self test and the running self test, counting the check-engine flashes each time.
New to flash codes? The OBD-I code reading guide walks through the paperclip method, code 12, and reading two-digit codes.
What does Ford code 646 mean?
646. Improper gear ratio in 2nd gear. This code sets when your vehicle computer detects improper gear ratio in 2nd gear.
Common causes of 646
- ATF level/condition
- Poor connection
- Wiring
- TR sensor
- Clutch slipping
- ECM
How to fix 646
- Check the transmission fluid level and condition, and look for slip under load
- Check the connector pins for corrosion, spread terminals or a poor fit
- Check the circuit wiring and harness for damage, chafing or a poor splice
- Test the TR sensor and replace it only if it fails its own test
- Test the ECM last, only after every circuit test has passed
- Clear the code, then retest to confirm the fix holds