139 No HO2S switch detected

How to read it

Code 139 reads as 1 flash, pause, 3 flashes, pause, 9 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.

What does Ford code 139 mean?

139. No HO2S switch detected. This code sets when your vehicle computer detects no ho2s switch detected.

Common causes of 139

• Intake leak/blocked
• Injectors
• HO2S
• AIR system
• Breather system
• EVAP system
• MAP sensor
• Ignition system
• ECM

How to fix 139

1. Check for an intake leak or a blockage
2. Check the injectors for a clogged or a leaking one
3. Test the HO2S and replace it only if it fails its own test
4. Check the AIR system for a stuck valve, a blocked passage or a dead pump
5. Check the breather system for a blockage
6. Check the EVAP system for a leak or a stuck valve
7. Test the MAP sensor and replace it only if it fails its own test
8. Check for ignition system
9. Test the ECM last, only after every circuit test has passed
10. Clear the code, then retest to confirm the fix holds