P078F
Transmission Mode Switch C Circuit
Generic OBD-II
Common symptoms
What you'll notice
- Inspect and clean the transmission mode switch contacts
- Replace the faulty transmission mode switch
- Check and repair any damaged wiring or connections related to the transmission mode switch
- Inspect and repair the transmission control module (TCM) if necessary
Common causes
Most likely first
- Faulty transmission mode switch
- Loose or corroded electrical connections to the transmission mode switch
- Damaged or worn wiring harness leading to the transmission mode switch
- Faulty transmission control module
- Low transmission fluid levels or contaminated fluid
How to diagnose & fix
Work through it in order
- The vehicle may experience shifting difficulties or may not shift at all.
- The transmission may stay in a single gear and not shift up or down.
- The engine may rev high without any increase in vehicle speed or acceleration.
- The vehicle may go into a limp mode, limiting the engine's performance or speed.
- The check engine light may illuminate on the dashboard.
Read & clear P078F yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.