P078F
Transmission Mode Switch C Circuit
General electrical fault on the third transmission mode select switch input C — used for additional drive modes (Sport, Snow, Tow/Haul, Eco) on transmissions that offer more than two operator-selectable shift maps. Without a valid input the TCM falls back to the default mode.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: medium
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
Common causes
Ordered by how often each turns out to be the cause.
- Open or shorted switch wireCommon
Parts involved
- Transmission Mode Switch C
- Wiring Harness
P078F is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.