P0979 code meaning
Shift Solenoid C Control Circuit Low
Powertrain system Generic code P09xx codes Check engine light
What P0979 means
Low-voltage fault, usually a short to ground, in the control circuit of shift solenoid C. GM uses it mainly on the Allison 1000 six-speed of its heavy pickups and chassis cabs: in 2016 a ground short of 0.22 ohm or less, stricter than the 0.42 ohm for SS1 and SS2, or an open of 200 kOhm or more counts after 10 samples (250 ms, Type A). From 2022 the open moves to P097C and P0979 needs an intrusive low-side test (120 ms, MIL after one trip). The Allison PS3 pressure switch test (P0872) also points to P0979 when its fault is electrical. Ford uses it for SSC short to ground beside P0760 open; on 6R80 and 6F, SSC drives the CB26 clutch. High side is P0980.
P0979 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0979
- MIL on after the first faulty trip
- Transmission holds a default gear
- Rough or delayed shifts involving the affected clutch
Common causes of P0979
- Most likelySolenoid C control wire shorted to ground
- PossibleInternal short of solenoid C inside the valve body
- PossibleOpen coil on GM 2016 Allison calibrations, which report opens under this code
- Less likelyFaulty low-side driver in the control module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Shift solenoid cWiring harnessTCM
What each part of P0979 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 9 | Area: transmission. |
| 79 | The exact fault in that area. |
What to do next
- Note P0979 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0979
What does P0979 mean?
P0979 means "Shift Solenoid C Control Circuit Low". Low-voltage fault, usually a short to ground, in the control circuit of shift solenoid C. It is a generic code, so it means the same on every make.
Is it safe to drive with P0979?
Drive gently and only to a workshop. With P0979 the car can go into limp mode (reduced power) to protect itself.
Will P0979 fail an emissions test?
Yes, it can. P0979 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0979 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will P0979 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.