Comprehensive diagnostic guide for OBD-II code P0979

Shift Solenoid C Low (Powertrain / Transmission Control)

What This Code Means

• Open-source code definition indicates P0979 corresponds to a low signal condition on Shift Solenoid C (often described as ). In practical terms, the transmission control system detects that the solenoid C control circuit is not receiving or delivering the expected signal.
• OBD-II basics categorize this as a Powertrain/Transmission (P-code) DTC. Powertrain codes are monitored by the vehicle's control modules (TCM/PCM) to ensure proper engine and transmission operation. When the monitor detects an out-of-spec condition, it stores a DTC and flags the malfunction light if warranted.
• The code is therefore a transmission-related electrical fault, specifically tied to the solenoid C circuit and its control signal, rather than a purely mechanical gear train fault (though an undiagnosed mechanical issue can mimic or accompany it).

Symptoms you're likely to see

• Transmission shifting problems in the affected ranges/gears, often in lower gears or in shifts where solenoid C is active in the valve body.
• Harsh, delayed, slipping, or failed shifts; in some cases the transmission may go into a limp/fail-safe mode to protect itself.
• Inconsistent or no shift in certain gears; a "no-shift" condition can occur if the solenoid is not being driven as commanded.
• Check Engine / MIL illumination with P0979 stored in the PCM/TCM.

Note: Symptoms can vary by make/model and by whether P0979 appears alone or alongside other shift-solenoid or transmission-related codes.

Probable Causes

Grounded in practical diagnostic experience and common industry patterns:

• Faulty Shift Solenoid C (coil or internal mechanical fault) - most common cause.
  • Likelihood: relatively high (roughly 40-55% in typical cases).
• Wiring harness issue or connector problem to Shift Solenoid C (pin damage, corrosion, loose connection, short to ground or to power).
  • Likelihood: moderate (roughly 20-30%).
• Transmission Control Module (TCM/PCM) signal processing, software, or internal fault affecting solenoid drive commands.
  • Likelihood: moderate (roughly 10-25%).
• Power supply or grounding issue to the solenoid circuit (battery/ignition power loss, chassis/engine ground problems).
  • Likelihood: smaller but nonzero (roughly 5-15%).
• Transmission mechanical/valve-body condition masquerading as an electrical fault (less common as a primary cause for P0979, but can coexist).
  • Likelihood: smaller (roughly 5-10%).

Diagnostic Approach

1) Confirm the code and pull freeze-frame data

• Use a scan tool to confirm P0979 is present and note any freeze-frame data (engine RPM, vehicle speed, transmission fluid temperature, gear state) at the time the code set.
• Check for additional transmission-related codes that may accompany P0979 (e.g., other shift solenoid codes) to guide the investigation.

2) Visual and mechanical inspection

• Inspect solenoid wiring and connectors for damage, corrosion, bent pins, or looseness at the solenoid C harness connector and any nearby harness routing.
• Inspect the transmission's electrical grounds and battery power feed to the TCM/PCM and the solenoid power supply circuit. Look for damaged insulation, chafed wires, or aftermarket wiring that could cause interference.
• Check transmission fluid level and condition (color, smell, contamination). Abnormal fluid can affect valve body operation and solenoid performance indirectly.

3) Electrical tests on the Shift Solenoid C circuit

• With vehicle safely supported and ignition on (do not run RPM tests with the engine while probing live circuits; follow shop manual safety procedures):
  • Verify 12V reference/power to the solenoid C control circuit when commanded (as specified by manufacturer). Confirm that the ground path is solid and not intermittent.
  • Measure resistance of Shift Solenoid C coil (at the solenoid connector with the harness disconnected, following the service manual's specified resistance range). Compare to the spec; if out of spec, the solenoid is suspect.
  • Check for short to ground or short to power on the solenoid control circuit. Inspect for parasitic current draw or code flaps that indicate wiring issues.
  • If the solenoid is not receiving the proper command or voltage when the TCM asks for it, the fault may be in the wiring, connector, or TCM.

4) Functional/bench test of Shift Solenoid C

• If possible, bench-test the solenoid (with the appropriate service/manual procedures) to confirm it actuates when a controlled voltage is applied and that it returns to idle when voltage is removed.
• Compare solenoid behavior with a known-good solenoid of the same type, if available.

5) Transmission control module (TCM/PCM) data and software

• Review live data for the solenoid C control signal (duty cycle, PWM command, or on/off state depending on the transmission). Look for abnormally low or no drive signal during typical shift events.
• Check for programming/updates or TCM calibration issues per the vehicle manufacturer's service information. Reprogramming or software updates can resolve misinterpretation of sensors or control commands.

6) Cross-check for related codes and symptoms

• If other shift solenoids or valve-body related codes exist, consider a broader valve-body or solenoid pack issue.
• Determine if the vehicle has a known TSB (Technical Service Bulletin) addressing P0979 or related solenoid control issues for your make/model.

7) Integrated road test after repairs

• After performing a suspected repair (solenoid replacement, harness/connector repair, or TCM reprogram), perform a controlled road test focusing on the shifting behavior around the gears where solenoid C is involved.
• Re-scan for codes after the test to confirm the fault is cleared and verify no new codes appear.

Reasonable next steps based on likely outcomes

• If solenoid C tests out of spec or fails the bench test, replace the Shift Solenoid C and re-test.
• If the solenoid tests good but the control signal is still abnormal, pursue wiring/connector corrections and verify power and ground integrity. If wiring tests pass, evaluate the TCM/PCM for faults or software issues.
• If all electrical tests pass but the trans experiences mechanical symptoms, consider broader internal transmission diagnostics or valve-body inspection as a follow-up, while keeping in mind P0979 is primarily an electrical fault indicator.

Safety Considerations

• Always follow the vehicle manufacturer's safety procedures when testing electrical circuits. Disconnect the battery as required before disconnecting sensors or solenoids in some vehicles.
• Support the vehicle securely when performing any under-vehicle work or transmission service. Use proper PPE and avoid contact with hot components.
• When working with the transmission electrical systems, be mindful of risk of short circuits, arcing, and potential damage to the vehicle's electronics.

Documentation and references

• Obd-II / DTC overview and the concept that DTCs are generated by powertrain control modules (TCM/PCM) for system monitoring. This is described in general terms under the OBD-II Diagnostic Trouble Codes sections.

• OBD2 CODE DEFINITIONS: Shift Solenoid C Low as the likely interpretation for P0979

Notes on conflicts or gaps

• do not give vehicle-specific wiring diagrams, exact resistance values, or precise factory test procedures for P0979. Where exact specs are needed, refer to the vehicle's service manual and the TCM/solenoid manufacturer data for your model.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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