Powertrain, generic · Transmission

P078F Transmission Mode Switch C Circuit

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

P078F is the circuit fault of a third transmission mode switch (C), after A (P071A–P071C) and B (P071D–P071F): its state is invalid or implausible. GM's transmission modules of 2023 and 2025 list the number only as a blocking condition for another check, without a description of their own; GM reports its drive mode selector today under P05D0–P05D3 (8-step resistor ladder, two trips). No manufacturer document available to us describes P078F itself. The mode then cannot be selected and the transmission stays on its normal schedule.

Likely causes

Switch or button faulty or sticking
Wire or connector open or shorted
Moisture or dirt in the centre console
Control module

Symptoms

  • Drive mode (sport, winter, tow) cannot be selected
  • Mode indicator missing
  • Transmission stays on the normal program

Repair estimate

DifficultyEasyDIY possible
Parts and labour€40–250
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

transmission mode switchswitch wiringtransmission control module

Related codes

P071AP071DP0790P05D0P05D1P05D2

References: gsi.ext.gm.com, gsi.ext.gm.com, law.cornell.edu.