P0797 is the companion code to P0796. Where P0796 means Pressure Control Solenoid C isn't applying enough pressure, P0797 means it's stuck in the fully-on position - continuously applying maximum pressure to a clutch pack that should be modulated or released. The result is typically harsh, abrupt shift engagement rather than the smooth modulated engagement the TCM intends.
What "Stuck On" Means for Shift Quality
A pressure control solenoid that's stuck on applies full hydraulic pressure to a clutch pack regardless of what the TCM commands. Instead of the controlled ramp-up in pressure that creates a smooth shift, the clutch engages with full force immediately - producing a jolt or clunk on upshift or downshift. Some vehicles enter limp mode when P0797 is active because the TCM detects that it cannot control clutch apply pressure.
Common Causes
- Solenoid valve stuck open: Varnish buildup from degraded ATF can hold the solenoid valve in the open position. A fluid flush sometimes clears mild cases - but if the varnish has hardened, solenoid replacement is needed.
- Mechanical solenoid failure: The solenoid's internal spring or valve seat wears or breaks, preventing it from closing when de-energized.
- Wiring short to power: A shorted wire between the TCM and solenoid that continuously supplies voltage even when the TCM tries to turn the solenoid off.
- TCM output driver failure: The TCM's internal driver circuit for that solenoid channel fails in the on state.
Symptoms of P0797
- Check engine light with P0797 stored
- Harsh, jolting upshift or downshift into specific gears
- Clunk or bang when transmission changes gears - may feel like a bump from behind
- Vehicle entering limp mode (locked into one gear)
- Companion codes: P0700, sometimes P0796 if the solenoid is intermittently stuck
Diagnosis Steps
Proper diagnosis for P0797 includes:
- Check fluid condition and level - varnish-contaminated fluid is often the root cause
- Resistance test the solenoid circuit - a stuck-open solenoid often shows normal resistance but fails the functional test
- Wiring inspection - check for shorts to power in the solenoid harness
- TCM output testing - verify the TCM is correctly commanding the solenoid off
Chicago Repair Costs for P0797
- Transmission fluid exchange (if varnish contamination is contributing): quoted after diagnosis, access check, and parts availability.
- Solenoid replacement: quoted after confirming whether the unit uses an individual solenoid or a complete solenoid pack
- Wiring repair: quoted after diagnosis, access check, and parts availability.
- TCM replacement (if output driver failure confirmed): depending on vehicle - TCMs are often remanufactured units
- Valve body service (if multiple solenoids are varnished): quoted after diagnosis, access check, and parts availability.
Free diagnostic at Chicago Transmission - 2450 N Lincoln Ave. Call (312) 452-5637, Monday - Friday 7:30am - 6pm, Saturday 8am - 2pm.
