Ford 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.

What does Ford code 646 mean?

Improper gear ratio in 2nd gear

Common causes of 646

• A 2nd gear ratio that does not match because clutch packs slip
• Low or worn fluid that lets 2nd gear slip
• A range sensor reading the wrong position in 2nd gear
• Worn or broken hard parts in the 2nd gear set

How to fix 646

1. Check the transmission fluid level and condition, and watch for slip in 2nd gear
2. Check the range sensor adjustment and the shift linkage
3. Check the connector pins and wiring for the 2nd gear shift solenoid
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