Ford 648 Improper gear ratio in 4th gear

How to read it

Code 648 reads as 6 flashes, pause, 4 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.

What does Ford code 648 mean?

Improper gear ratio in 4th gear

Common causes of 648

• A 4th gear ratio that does not match because clutch packs slip
• Low or worn fluid that lets 4th gear slip
• A range sensor reading the wrong position in 4th gear
• Worn or broken hard parts in the 4th gear set

How to fix 648

1. Check the transmission fluid level and condition, and watch for slip in 4th gear
2. Check the range sensor adjustment and the shift linkage
3. Check the connector pins and wiring for the 4th 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