OBD-II trouble code
P0799: Pressure Control Solenoid "C" Intermittent
The fault comes and goes, which means you cannot find it with a meter while it is behaving. What you can do is capture it — freeze frame, min/max recording, and a wiggle test with the fluid hot.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $1,600
- DIY difficulty
- Shop recommended
What does P0799 mean?
P0799 is the intermittent member of the pressure control solenoid "C" family, and it is a fundamentally different diagnostic problem from its siblings rather than a milder version of them. The other codes in the family describe a condition the module can still see when you go looking: a circuit that is open now, a solenoid that is stuck now, a pressure that is wrong now. This one describes something that was true and then stopped being true. By the time the vehicle is on a lift with a meter across the connector, the circuit is very likely reading perfectly.
That single fact should reorganise the whole approach. Static testing is nearly worthless here. A resistance measurement taken while the fault is absent proves only that the fault is absent, and the temptation to conclude "tests fine, must have been a glitch" is exactly how an intermittent gets cleared, handed back, and returns three weeks later. The productive work is capture rather than measurement — reading the freeze frame the module stored at the moment the code set, setting up a min/max recording on the solenoid circuit and driving until it trips, and manipulating the harness and connector by hand while watching live data for a momentary drop-out.
Temperature is the variable that turns most of these from unfindable into findable. Almost everything that fails intermittently in a transmission solenoid circuit fails as a function of heat. A solenoid coil whose windings are breaking down has higher resistance hot than cold and may only fall out of specification once the fluid is at operating temperature. Insulation that has chafed against a case rib softens and expands when hot, so a conductor that clears a sharp edge in the morning touches it after forty minutes on the motorway. Connector terminals that have lost tension make adequate contact cold and lose it as the plastic body relaxes. Look at the fluid temperature recorded in the freeze frame — if it is high, that is a lead, not a coincidence, and the road test needs to be long enough to get there.
The other cause worth naming specifically is terminal tension, because it is the classic intermittent and it hides from every test that does not disturb it. Female terminals in a connector are sprung, and a terminal that has been spread — by probing with too large a probe, by repeated disconnection, or by vibration over years — still touches the male pin firmly enough to pass a continuity test on the bench. Under vibration, with heat, under the current the solenoid actually draws, it lets go momentarily. The tell is that a wiggle test on the connector reproduces the fault when nothing else does, and the repair is a terminal, not a solenoid.
A note on urgency, because intermittent faults invite being ignored. Between events the vehicle drives normally, and the code may even self-clear, which makes it feel optional. It is not. Intermittent electrical faults in a transmission harness get worse rather than better — chafe deepens, corrosion spreads, a spread terminal never re-tensions itself — and the endpoint is a permanent fault that arrives without warning, quite possibly with the transmission dropping into failsafe somewhere inconvenient. The window in which this is a cheap connector repair is now, while it is still intermittent.
Common causes
- Connector terminal that has lost spring tension and breaks contact under vibration or heat
- Solenoid coil windings breaking down, drifting out of specification only once the fluid is at operating temperature
- Wiring chafed against a case rib or bracket, making momentary contact when the harness expands with heat
- Corrosion inside the transmission case connector allowing resistance to vary with temperature and moisture
- Internal case harness fatigued where it flexes with the valve body
- Debris in the fluid intermittently interfering with the solenoid spool without holding it
- Fluid level marginal, so the pump aerates on cornering or hard braking and pressure control momentarily loses authority
- Poor module ground allowing the measured circuit voltage to shift under electrical load
- Damaged pin in the module connector after a previous repair or module swap
- Aftermarket wiring or a prior harness repair with a joint that has begun to fail
Symptoms
- Occasional harsh or delayed shift that does not repeat on demand
- Check engine light that comes on, then goes out again after a few drive cycles
- Transmission briefly entering failsafe and recovering after a restart
- A single unexplained shift flare on an otherwise normal journey
- Symptoms appearing only after the vehicle has been driven for half an hour or more
- Fault more likely on rough roads or over speed bumps than on smooth ones
- Stored code with no drivability complaint the driver can describe
- Shift quality that changes between a cold morning start and the drive home
- Other pressure control or solenoid codes appearing sporadically alongside this one
Diagnostic steps
- 1.Read and save the freeze frame before clearing anything. Fluid temperature, road speed, engine load and gear at the moment the code set are the most useful data you will get on an intermittent fault.
- 2.Do not treat a clean resistance reading as a result. A static test taken while the fault is absent confirms only that the fault is absent, which you already knew.
- 3.Set up a min/max or peak-hold recording on the solenoid circuit and drive the vehicle until the code sets, rather than testing at idle on a lift.
- 4.Get the fluid to full operating temperature before testing. Most intermittent solenoid faults are heat-dependent and will not appear on a cold vehicle.
- 5.Perform a wiggle test on the transmission case connector and the harness leading to it while watching live data, and repeat it hot. Reproducing the fault by hand localises it faster than any other method.
- 6.Inspect the female terminals in the connector for spread or discoloured contacts. A terminal that has lost tension passes a continuity test and still fails under vibration and load.
- 7.Check the harness where it runs against case ribs, brackets and the exhaust for chafe marks, paying attention to sections that move as the powertrain rocks.
- 8.Check fluid level and condition properly at the specified temperature, since a marginal level can aerate under cornering and produce an intermittent pressure fault with no electrical cause at all.
- 9.Measure voltage drop on the module ground path under load rather than checking continuity, because a marginal ground shifts the whole circuit reading when other loads switch on.
- 10.Confirm which physical solenoid is designated "C" on this specific transmission from service data before ordering anything, as the lettering is not consistent between designs.
Repair cost
$120 – $1,600
The good outcome is a connector: repairing or replacing corroded or spread terminals runs $120 to $400 and fixes the problem permanently. A harness repair where a conductor has chafed is $200 to $600 depending on access and whether the pan has to come down. A single pressure control solenoid is $250 to $700 fitted; a full solenoid pack or valve body assembly is $600 to $1,600. A fluid and filter service, if a marginal level turns out to be the cause, is $150 to $400. Expect to pay $150 to $300 for diagnosis on this code and consider it money well spent — intermittent faults are where guessing at parts gets expensive fastest, because a replaced solenoid that was never the problem still tests fine afterwards.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with transmission shift solenoid replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.