What P0977 means
High-voltage fault in the control circuit of shift solenoid B.
How the engine computer detects it
GM six-speeds put an open or a power short of the mode 3 multiplex valve here, 1.2 of 1.5 s; the 2016 table sets it without MIL, the 2019 table with MIL after one trip. The Saab 9-4X 6T70 treats it more severely than P0976: 2 s, Type A, high-side driver off, only reverse and 5th gear left, no converter lockup. Ford uses it strictly for SSB short to power, as in its 2012 and later summaries. GM's Allison 1000 checks for 0.39 ohm or less to a supply over three samples (75 ms in 2016, 60 ms in 2023). FCA 9-speed and Aisin uses are not documented for this code. Ground-side faults are P0976.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Limp mode with only reverse and one high forward gear
- Weak acceleration from rest
- MIL or transmission warning, depending on calibration
Common causes
- Open solenoid B coil or wire on GM six-speeds, where opens report as high Most likely
- Control wire shorted to battery or a switched supply Possible
- Damaged pins or seals at the transmission pass-through connector Possible
- Faulty driver stage in the control module Less likely
How to diagnose P0977
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: shift solenoid b, wiring harness, tcm.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 9
- Transmission
- 77
- Specific fault
Questions drivers ask
Can I drive with code P0977?
If the car shifts normally, drive gently to a shop. If it's stuck in one gear, slips, shifts harshly or won't move out of Park, don't drive it: have it towed.
Will my car pass an emissions inspection with P0977?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.