21 Engine coolant temperature sensor out of self test range
How to read it
Code 21 reads as 2 flashes, pause, 1 flash 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 21 mean?
21. Engine coolant temperature sensor out of self test range. This code sets when your vehicle computer detects engine coolant temperature sensor out of self test range.
Common causes of 21
- Wiring
- ECT sensor
- ECM
How to fix 21
- Check the circuit wiring and harness for damage, chafing or a poor splice
- Test the ECT 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