638 Transmission fluid temperature sensor circuit below minimum voltage
How to read it
Code 638 reads as 6 flashes, pause, 3 flashes, pause, 8 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 638 mean?
638. Transmission fluid temperature sensor circuit below minimum voltage. This code sets when your vehicle computer detects transmission fluid temperature sensor circuit below minimum voltage.
Common causes of 638
- ATF level
- Wiring short circuit to ground
- TFT sensor
- ECM
How to fix 638
- Check the transmission fluid level and condition, and look for slip under load
- Check the circuit wiring for a short to ground
- Test the TFT 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