Ford 645 Improper gear ratio in 1st gear

How to read it

Code 645 reads as 6 flashes, pause, 4 flashes, pause, 5 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 645 mean?

Improper gear ratio in 1st gear

Common causes of 645

• A 1st gear ratio that does not match because clutch packs slip
• Low or worn fluid that lets 1st gear slip
• A range sensor reading the wrong position in 1st gear
• Worn or broken hard parts in the 1st gear set

How to fix 645

1. Check the transmission fluid level and condition, and watch for slip in 1st gear
2. Check the range sensor adjustment and the shift linkage
3. Check the connector pins and wiring for the shift solenoids
4. Test the range sensor and replace it only if it fails its own test
5. Test the PCM last, only after every circuit test has passed
6. Clear the code, then retest to confirm the fix holds

Source

Qualified source review