P0977

Shift Solenoid B Control Circuit High

P0977 is a generic OBD-II powertrain diagnostic trouble code: Shift Solenoid B Control Circuit High. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0977
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on, possible limp mode)
Need P0977 disabled?
RaceTune permanently disables any OBD-II trouble code on supported ECUs — for motorsport, off-road, and export use.
→

What P0977 means

P0977 is set when the TCM detects a higher-than-expected voltage on the Shift Solenoid "B" (Clutch Pressure Control Solenoid B) control circuit. In normal operation the TCM's low-side PWM driver pulls the circuit toward ground in a controlled duty cycle; a "high" circuit fault means the driver is measuring unexpected voltage on the line, indicating a short-to-battery on the control wire, a failed TCM driver transistor that cannot switch the output low, or a solenoid internally shorted in the high-resistance path.

P0977 is to Solenoid B what P0974 is to Solenoid A — both are "high" circuit conditions pointing to an unintended voltage source in the driver circuit. Because the TCM can no longer modulate clutch apply pressure on the Solenoid B circuit, it disables that driver output and sets fail-safe mode. This is a protective measure against hydraulic pressure runaway that could burn a clutch pack within seconds of unrestricted engagement.

Common platforms include GM 6L80/6T70/8L90, Ford 6R80/10R80, ZF 6HP26/8HP45, and Mercedes-Benz 722.9. The repair approach mirrors P0974: inspect for harness shorts to a power supply first, test the TCM driver if no external short is found, and bench-test the solenoid coil before replacing any major component.

Common causes

Most-frequently reported root causes when P0977 is logged.

  • 1
    Short-to-battery on the Solenoid B control wire where harness insulation is worn through against a 12 V supply.
  • 2
    Shorted low-side driver transistor inside the TCM holding the control output permanently high.
  • 3
    Internally shorted solenoid coil creating an unintended voltage path that the TCM reads as a circuit high condition.
  • 4
    Bridged connector terminals with an adjacent power pin due to corrosion, pin spread, or terminal damage.
  • 5
    Insulation damage from heat soak near the exhaust or turbocharger allowing the control wire to contact power-supply wiring.
  • 6
    Water or fluid ingression into the transmission connector corroding adjacent pins and creating cross-circuit paths.
  • 7
    Prior wiring repairs using incorrect wire gauge or unprotected splices that have developed intermittent high-voltage contact.

Symptoms drivers notice

MIL / Check Engine Light illuminated, often with a separate transmission warning light.
Transmission locked in limp mode, typically 2nd or 3rd gear, across all driving conditions.
Inability to achieve normal upshifts; vehicle revs high before any shift occurs.
Harsh or jerky gear engagements, particularly when moving from Park or Neutral into Drive.
Torque converter clutch disabled, resulting in noticeably elevated RPM at cruising speed.
Possible transmission overheating indication if the vehicle is driven extended distances in limp mode.

How to diagnose P0977

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs with a scan tool; note whether P0976 (Low) is also stored alongside P0977 (High), as both codes together strongly suggest a TCM internal driver fault rather than an external wiring short.
  2. 2
    Inspect the transmission harness for insulation damage, heat exposure near exhaust routing, or contact between the Solenoid B control wire and any adjacent 12 V power wire.
  3. 3
    Disconnect the transmission harness connector and, with the ignition on, measure voltage on the Solenoid B control terminal; any reading above 1 V with the harness unplugged confirms an external short-to-power.
  4. 4
    With the harness unplugged and ignition off, measure resistance of the Solenoid B control wire from the TCM connector to chassis ground — a near-zero reading indicates the TCM driver transistor is shorted internally.
  5. 5
    Inspect all connector pins for bridging, corrosion, or mechanical damage that could create electrical contact between the control pin and an adjacent power supply pin.
  6. 6
    If external wiring is confirmed fault-free, bench-test Solenoid B resistance directly at the solenoid body; an out-of-specification reading confirms solenoid replacement is needed.
  7. 7
    Verify transmission fluid level and condition as part of the full service; replace if burnt or contaminated before returning the vehicle to service.

Related powertrain codes

Frequently asked questions

What is the difference between P0976 (Low) and P0977 (High) on Solenoid B?

P0976 means the circuit voltage is below the minimum — caused by an open wire, short to ground, or open solenoid coil. P0977 means the circuit voltage is above the maximum — caused by a short to battery voltage or a shorted TCM output driver. Low faults usually indicate broken wiring; High faults usually indicate an unintended connection to a power source.

If I see P0976 and P0977 at the same time, what does that mean?

Simultaneous Low and High codes on the same solenoid circuit almost always indicate a failed TCM driver stage rather than a wiring fault. The driver cannot swing between low and high states because its transistor has failed in an intermediate condition. In this case the TCM itself likely needs replacement.

Can P0977 be an intermittent fault?

Yes, particularly when caused by a harness wire that only contacts a power supply under vibration or thermal expansion. If the code does not return immediately after clearing, conduct a thorough physical inspection of the harness routing while moving the loom by hand to reproduce the contact.

Does P0977 require immediate attention or can I wait?

The transmission will operate in limp mode, which protects the hardware, but extended limp-mode driving raises fluid temperatures and accelerates wear on the locked-in gear's clutch pack. Repair within a few days is advisable; avoid motorway driving until the fault is resolved.

Disabling P0977 in software

RaceTune can permanently disable P0977 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

Got P0977 in your scan?

Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.

Upload your file