P0977
PowertrainShift Solenoid "B" Control Circuit High
The transmission control module fires shift solenoid B by switching its control circuit, and it watches the voltage on that line to confirm the command landed. When it sees the voltage sitting high when it shouldn't be, it logs P0977 and usually drops the gearbox into limp mode to protect itself. For you that means the auto box has spotted an electrical fault in one of the solenoids that handles gear changes, and it's gone into a safe state rather than risk damaging itself. It's an electrical complaint at heart, not always the solenoid itself.
ⓘ Information only. This page provides general educational information about fault code P0977. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0977 mean?
P0977 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Shift Solenoid "B" Control Circuit High.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, sometimes with a separate transmission or gearbox warning depending on the car
- • Harsh, delayed or flaring gear changes, especially noticeable pulling away from a stop
- • Gearbox locked in one gear, often third, which is the classic limp mode behaviour
- • Limited power and the car feeling like it won't rev out properly
- • Jerky or hunting shifts when the box can't settle on a ratio
- • Reluctance or a clunk when selecting drive or reverse
Possible causes
Causes commonly associated with P0977, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Corroded or damaged connector at the transmission, water and road salt get in and play havoc with the low-voltage signal. This is the first thing to rule out before spending money
- 2. Failed shift solenoid B with an internal electrical fault, the winding goes open circuit or shorts and the TCM sees a high voltage
- 3. Chafed or broken wiring in the loom between the TCM and the solenoid, often where it passes a heat source or rubs on a bracket
- 4. Internal transmission harness breaking down from years of heat soak in the fluid, common on higher-mileage autos
- 5. Low or burnt transmission fluid, which won't directly cause a circuit-high reading but goes hand in hand with solenoid trouble and shouldn't be ignored
- 6. Faulty valve body where the solenoid is mounted, seen on some CVT and conventional autos
- 7. Poor earth or a supply fault upstream of the solenoid circuit
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the codes and freeze frame, then note exactly when it logged. A code that sets cold versus hot points you in very different directions
- 2. Unplug the main transmission connector and check it properly for green corrosion, bent pins and signs of water getting in. A surprising number of these are fixed right here
- 3. Measure shift solenoid B resistance at the connector, typically somewhere around 11 to 15 ohms at room temperature though it varies by box, so check the figure for your model. Open circuit or a wildly high reading condemns the solenoid
- 4. Check continuity and condition of the wiring from the TCM through to the solenoid, wiggling the loom while watching for changes if you can
- 5. Confirm the fluid level and condition while you're under there, dark or burnt fluid is a warning the box has had a hard life
- 6. If the external wiring and connector are clean and the resistance is out of spec, the fault is inside, which means dropping the pan to get at the solenoid pack or harness
Common questions about P0977
How do I work out whether it's the solenoid or just the wiring? +
Start at the transmission connector, because a corroded or wet plug throws this code far more often than people expect and costs nothing but your time to check. Pull it apart and look for corrosion or moisture. If the pins are clean, measure the solenoid resistance through that connector, you're looking for a sensible reading in the region of 11 to 15 ohms depending on the box. An open circuit or a reading way off spec means the solenoid or internal harness has failed. A good reading with clean wiring usually points to a connector or earth issue you can sort externally.
Can I sort this myself or is it a garage job? +
The cheap end is fixable at home. Cleaning or repinning a corroded transmission connector, checking earths and topping up or changing the fluid are all doable on a driveway with basic tools. The solenoid itself is the problem. On most autos it lives inside the box behind the pan, and getting to it means dropping the pan, draining fluid and refilling with the exact spec your gearbox wants. Get the fluid wrong and you cause more damage than the original fault, so if you're not set up for that, hand it to an auto box specialist rather than a general garage.
If I clear the code does it stay gone? +
Clearing it might give you a couple of minutes of normal shifting, but if the circuit fault is still there the TCM will see the high voltage again and relog it, often before you've left the road. A code that comes straight back is telling you the fault is live and real. The only time clearing it sticks is after you've actually fixed the cause, say cleaned a corroded plug. Resetting and hoping is a waste of time on this one.
What happens if I just keep driving on it? +
Most cars in limp mode are still driveable enough to get home or to a garage, usually stuck in one gear with no real power. The danger is doing that for weeks. Running an auto box that can't shift properly builds heat, and heat is what cooks transmission fluid and wrecks clutch packs. You can turn a cheap connector repair into a full gearbox rebuild costing four figures by ignoring it. Drive it gently to get it looked at, then stop using it for daily trips until it's fixed.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →