Faulty mode switch or selector assembly (worn contacts, sticking button, spilled drink); Damaged, corroded or loose wiring or connector in the switch circuit; Short to earth, short to supply or open circuit in the signal, reference or earth wire; Low battery voltage or poor control unit supply or earth causing implausible readings; Fault in the gear selector or shifter module that contains the switch; Faulty transmission or powertrain control unit, or outdated software where the manufacturer lists it as a known cause.

P078F

Transmission Mode Switch C Circuit.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
7: AreaTransmission
8F: FaultThe specific fault in that area

What it means

P078F is the generic code for a fault in the Transmission Mode Switch 'C' circuit. The transmission or powertrain control unit monitors the switch signal that the driver uses to select a drive mode, such as sport, economy, snow/winter or manual mode. It set the code because the signal was out of range, stuck, intermittent or did not agree with other inputs. Which switch is 'C' and how it is wired depends on the vehicle, so check manufacturer data for the model.

Symptoms

Transmission or powertrain warning light on
Drive mode selector not responding or stuck in one mode
Transmission defaults to a fixed shift program, such as limp-home or standard mode
Harsh, delayed or unexpected gear changes
Paddle shift or manual mode not working
Mode indicator on the dash flickering or incorrect

Possible causes

Faulty mode switch or selector assembly (worn contacts, sticking button, spilled drink)likely
Damaged, corroded or loose wiring or connector in the switch circuitlikely
Short to earth, short to supply or open circuit in the signal, reference or earth wirepossible
Low battery voltage or poor control unit supply or earth causing implausible readingspossible
Fault in the gear selector or shifter module that contains the switchpossible
Faulty transmission or powertrain control unit, or outdated software where the manufacturer lists it as a known causerare

How to diagnose

Read all stored and pending codes and record the freeze frame data, noting any related communication or battery voltage codes.
Check the battery condition and charging voltage, as low voltage can cause false switch faults.
Operate the mode switch through every position and watch the live data to see whether the switch state changes and matches the dash display.
Inspect the switch, selector and connectors for moisture, spilled liquid, corrosion, bent pins and damage.
Check the harness along its route for chafing, then wiggle-test the connector while watching live data for dropouts.
Using the manufacturer wiring diagram, test the supply, signal and earth for continuity and for shorts to earth or supply, and check the switch resistance or voltage in each position.
Check the control unit supply and earth connections and look for any software updates or technical bulletins for the model.
Clear the code and road test to see whether it returns.

How to fix

Replace the mode switch or selector assembly if it fails testing.
Repair or replace damaged wiring, and clean or replace corroded or loose connectors.
Restore faulty supply or earth connections and correct any battery or charging problems.
Replace the shifter or selector module if the switch is not serviced separately.
Update or reflash the control unit software only if the manufacturer lists it as a fix, and replace the unit only after all other causes are ruled out.

Can I keep driving?

Medium severity

The car can usually be driven carefully because the transmission normally falls back to a default mode, but shifting may be harsh or limited. Get it checked soon, and stop driving if the gearbox goes into limp mode or shifts unpredictably.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.