P0979

Code

Generic P — Powertrain

Shift Solenoid C Control Circuit Low

Brand: Generic

Type: P — Powertrain

Views: UK: 25 EN: 54 RU: 35

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Page language: EN

Causes

Short to ground in the shift solenoid C control wiring
Corroded/damaged connector or poor pin contact at the solenoid
Failed shift solenoid C (internal short or open)
Failed/shorted driver transistor inside the TCM/PCM
Blown fuse or relay feeding the transmission solenoids
Poor battery/ground connections or low system voltage

Symptoms

Check Engine / MIL illuminated (transmission fault)
Harsh, delayed, or missing shifts; transmission may be stuck in one gear
Transmission limp-in mode (reduced shift availability)
Erratic shift behavior or frequent gear hunting
Possible transmission fluid leak at connector area (if wiring damaged)

What to check

Read and record freeze frame and live data with a capable scan tool; note drive/idle conditions when code set
Visual inspection of wiring and connector for damage, corrosion, pin push-out, or fluid contamination
Check related fuses and transmission control relay for proper operation
Backprobe solenoid connector and monitor voltage with key ON / engine OFF and while commanding solenoid ON/OFF from scan tool
Measure solenoid coil resistance (with connector disconnected) and compare to specification or known-good value
Perform a wiggle test on harness while monitoring signal or data to reproduce fault

Signal parameters

Typical solenoid coil resistance: commonly 5–30 ohms (manufacturer-specific)
Normal resting voltage at solenoid power feed: ~12 V (ignition ON) if circuit is battery/switched power fed
Control/driver pin voltage (not commanded): close to battery voltage (~12 V) if power-fed and driver high‑impedance
Control/driver pin voltage (commanded ON): near 0 V (ground) if PCM grounds the circuit to activate solenoid
PWM control: some systems modulate duty (tens to hundreds of Hz) — duty cycle varies with commanded pressure
Low circuit fault: control pin measured near 0 V when solenoid is NOT commanded (indicates short to ground)

Diagnostic algorithm

Retrieve codes and freeze-frame data; confirm P0979 is current and note operating conditions.
Visually inspect harness and connector for damage, corrosion, pin deformation, or fluid contamination; repair any obvious issues.
With ignition OFF, disconnect solenoid C connector and measure coil resistance across solenoid terminals. Compare to spec or another solenoid. If open/infinite or shorted to ground, replace solenoid.
Check continuity from the TCM control pin to the solenoid connector pin (with harness disconnected) for short to ground and for open circuits. Expect no continuity to chassis ground when the connector is disconnected.
With ignition ON (engine OFF), backprobe the harness at the solenoid connector: verify battery/feed voltage on the power side and check the control side. Command the solenoid ON using a scan tool and observe the control pin transition to near 0 V (or PWM). If control pin is stuck low when NOT commanded, suspect a short to ground or bad TCM driver.
If wiring and solenoid check good but control pin remains low when disconnected, suspect TCM/PCM driver fault. Confirm with manufacturer procedures; consider substituting known-good module if applicable and supported.
After repair (wiring/solenoid/module), clear codes and road test to verify the fault doesn’t return; re-scan for pending codes.

Likely causes

Chafed wire contacting chassis or transmission case (short to ground)
Corroded terminal at solenoid harness (low voltage/poor contact)
Faulty solenoid coil with short to ground
Failed TCM/PCM output driver

Fault status

⚠️ Status

The transmission control module has detected that the voltage on the Shift Solenoid C control circuit is below the allowed threshold for a set time and flagged a low control circuit condition (possible short to ground or driver/solenoid fault).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours