P0979: Shift Solenoid C Control Circuit Low

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.

Symptoms

• MIL on after the first faulty trip
• Transmission holds a default gear
• Rough or delayed shifts involving the affected clutch

Most likely causes

• Solenoid C control wire shorted to ground
• Internal short of solenoid C inside the valve body
• Open coil on GM 2016 Allison calibrations, which report opens under this code
• Faulty low-side driver in the control module

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the fluid level and condition according to the manufacturer
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Check the transmission connector
4. Read the data with a transmission-compatible scanner
5. Measure the solenoid valve circuit
6. Leave opening the transmission to a specialized repair shop

Good to know

• Repair difficulty: hard
• It can trigger limp mode.
• It affects emissions.

What each character of P0979 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• 9: Transmission
• 79: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0979?

Drive as little as possible and gently. If the transmission slips or stays in one gear, go slowly to a repair shop or call for a tow truck.

What causes code P0979?

The most common causes are: solenoid C control wire shorted to ground; internal short of solenoid C inside the valve body; open coil on GM 2016 Allison calibrations, which report opens under this code.

Related codes

• P0980Shift Solenoid C Control Circuit High
• P097CShift Solenoid C Control Circuit/Open
• P0978Shift Solenoid C Control Circuit Range/Performance
• P0760Shift Solenoid C
• P0872Transmission Fluid Pressure Sensor/Switch C Circuit Low

Sources

• gsi.ext.gm.com: 16OBDG08 Transmission Diagnostics
• gsi.ext.gm.com: 23OBDG06B
• fordservicecontent.com: OBD Operations Summary Gasoline 2020 2023

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.