How do I tell whether the solenoid is stuck or the wiring is holding it on?

On a Trax or Encore 6T40, DTC P0796 or P0797 Circuit/System Testing compares line pressure with the engine idling, then tests the removed Q8 after the ignition is off. Disconnecting the solenoid in the car and watching pressure is not that chart. For pressure control solenoid valve 3 on that unit, DTC P0969–P0971 says the low side driver switches the coil and a high side driver supplies power, so a short to ground on the control wire can hold the coil energized. The same description says the solenoid is normally high: more current lowers fluid pressure. On any other unit, use that unit's service information and its wiring diagram for the driver side.

Why is stuck on treated as different from stuck off?

Because stuck off leaves the circuit where the transmission is designed to leave it when it loses power, so the unit generally has a sensible fallback. Stuck on leaves a circuit commanded that the module cannot cancel, so the controller has lost the ability to manage that pressure at all and has to work around it. In practice that tends to mean a more consistent, less driving-dependent symptom, and a stronger case for getting it looked at promptly.

It went wrong overnight rather than gradually. Is that meaningful?

It is a genuine clue. Contamination faults creep in as varnish builds, so they usually start intermittent and slowly get worse. A fault that appeared abruptly is more consistent with something arriving — a piece of debris reaching a valve and wedging it, or an electrical driver failing short. Mentioning that timeline to whoever diagnoses it is worth more than most of what a scan tool will report.

Can I keep driving it?

Short, gentle trips are usually possible, but this is not one to leave. Pressure that cannot be reduced means clutches applying harder than intended, and pressure that cannot be raised means they slip. Both generate heat, and heat degrades the fluid, which puts more debris into the valve body — the same debris that most likely caused the problem. Avoid towing and heavy loads, and get the pressure test done before the situation compounds.

OBD-II trouble code

P0797: Pressure Control Solenoid "C" Stuck On

The module can turn this circuit on and cannot turn it off. That is a loss of authority rather than a loss of function. Which short can hold this coil on, and which hydraulic test applies, come from this vehicle's wiring diagram and its service information.

Medium severityPowertrainTransmission / Pressure ControlDrivable short-term

Quick facts

System: Powertrain
Category: Transmission / Pressure Control
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $150 – $1,400
DIY difficulty: Shop recommended

One of 26 pressure control solenoid codes

P0797 sits in a grid: one axis is which solenoid the letter refers to, the other is what went wrong with it. Each cell is a genuinely different fault with its own diagnosis — but codes from the same row or column often appear together, and what a pairing means is usually the fastest way in. The hub page lays out all 26 and explains how to read them against each other.

See all pressure control solenoid codes →

Browse every code in P0700–P0799, or start from the full code library.

What does P0797 mean?

P0797 and P0796 are often treated as two flavours of the same fault, and they are not. Stuck off leaves a circuit sitting in its de-energised state, which is the state the transmission is designed to fall back to when everything is switched off. Stuck on leaves a circuit commanded in a way the module cannot cancel. That is a different kind of failure: the module has not lost a function, it has lost authority. Whatever this pressure circuit is doing, it will keep doing regardless of what the controller wants, and the transmission has to operate around it.

Separating a stuck valve from a control-side fault still has to come from the procedure for the unit in the bay. On a Chevrolet Trax or Buick Encore with the 6T40 (RPO MH8 or MHB), the chart titled DTC P0796 or P0797 covers pressure control solenoid valve 3, and the note at the top says those two codes set from a hydraulic or mechanical condition. Its Circuit/System Testing compares transmission line pressure while the engine is idling at normal operating temperature with the tables Line Pressure Check and Solenoid Valve Pressure (Gen 2). The ignition is then turned off, the Q8 control solenoid valve assembly is removed, and the Control Solenoid Valve and Transmission Control Module Assembly Solenoid Performance Test is run. Disconnecting the solenoid in the vehicle and watching a gauge is not a step on that chart. Driver polarity for the same solenoid is in DTC P0969–P0971, Circuit/System Description, in that book: the TCM varies current by how long the low side driver is on, and it feeds the coil through a high side driver. The description calls the solenoid normally high, so raising current opens the exhaust and lowers fluid pressure. A short to ground on that low-side control wire can keep the coil energized while the high side stays enabled. An open, or a short to voltage, is what that chart names P0971. A unit those charts do not cover is diagnosed from its own wiring diagram, for the driver side and for the short that can hold that coil on, and from its own service information for the hydraulic test.

The onset history is also worth asking about, because it tends to differ from the stuck-off pattern. Stuck-off faults usually creep in as varnish accumulates, so the complaint starts as an occasional oddity and gradually becomes constant. Stuck-on faults are more often abrupt — a piece of debris arriving at a valve and wedging it, or an output driver failing short. An owner who says it was completely fine on Tuesday and wrong on Wednesday is describing something that arrived rather than something that wore in, and that shifts the suspicion toward debris and toward the electrical driver.

As with its sibling, what the symptom feels like depends on whether this transmission uses normally-high or normally-low pressure control solenoids, so establish that before interpreting the complaint. And as with every code in this family, the code names a behaviour rather than a part: low fluid, a clogged filter, a scored bore and contamination can all hold a valve where it should not be, with a perfectly good solenoid bolted to it.

Common causes

Debris wedged in the valve bore holding the spool in its applied position
Solenoid seized internally, unable to return when the command is removed
Module output driver failed short, keeping the solenoid energised regardless of command
A short on the switched side that can hold this coil on: to ground on a Trax or Encore 6T40 pressure control solenoid valve 3 low side driver, and whichever side this vehicle's wiring diagram shows on any other unit
Varnish or scoring in the bore preventing the return spring from moving the spool
Broken or fatigued return spring inside the solenoid
Contaminated fluid carrying debris into the valve body after wear elsewhere
Valve body damage from a previous repair or from a dropped valve during service

Symptoms

Shift quality wrong in a consistent direction that does not change with driving style or load
Harsh, slamming engagement or persistent slipping, depending on whether the solenoid is normally high or normally low
Symptom that appeared suddenly rather than gradually worsening
Transmission overheating
Limp mode holding a fixed ratio
Check engine light with P0797 stored, often with P0700 alongside
Engagement into drive or reverse feeling abrupt

Diagnostic steps

1.Ask how it started. A sudden onset points at debris or at an electrical driver failure; a gradual one points back toward contamination and the stuck-off failure mode instead.
2.Check fluid level and condition, and look up whether this transmission's pressure control solenoids are normally high or normally low so the symptom can be interpreted correctly.
3.On a Trax or Encore 6T40, follow DTC P0796 or P0797 Circuit/System Testing: engine idling at normal operating temperature, line pressure against Line Pressure Check and Solenoid Valve Pressure (Gen 2), then ignition off, remove the Q8 control solenoid valve assembly, and run the solenoid performance test. That chart does not disconnect the solenoid in the vehicle and watch pressure. Any other transmission follows its own service information.
4.Read this vehicle's wiring diagram for the driver side and check the short that can hold this coil on. On the Trax and Encore 6T40 pressure control solenoid valve 3, DTC P0969–P0971 Circuit/System Description says the TCM switches a low side driver and feeds the coil through a high side driver, so a short to ground on that control wire can hold the coil energized while the high side is enabled. A short to voltage on that wire is the P0971 fault in the same chart. The description also says the solenoid is normally high, so more current lowers fluid pressure. A diagram that switches the feed uses the short that diagram shows.
5.Resistance-check the solenoid and inspect the connector for corrosion and fluid intrusion.
6.Drop the pan and inspect the fluid and magnet for the debris that most often causes a valve to wedge.
7.Once accessible, remove the solenoid and check the spool moves freely by hand in a clean bore. A spool that will not move is the answer; a spool that moves freely sends the search back to the control side.

Repair cost

$150 – $1,400

A fluid and filter service at $150 to $350 addresses the contamination that lodges debris in a valve, though it is a less reliable fix here than on the stuck-off code because a wedged valve does not always free itself. A solenoid replacement is $250 to $700 fitted. A valve body is $600 to $1,400. The scenario worth identifying early is the electrical one: if the fault is a short on the switched side of the control wire or a failed module driver, the repair is a wiring or module job and no amount of transmission work will touch it. Sort that branch from this vehicle's wiring diagram and its service procedure before the pan comes off.

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DIY vs shop

Leave this one to a qualified shop. The barrier here is internal hydraulic and mechanical work, fluid handling at temperature, and an adaptive relearn afterwards without which a correct repair still shifts badly. DIY attempts often produce a more expensive problem than the original code.