OBD-II trouble code
P0779: Pressure Control Solenoid "B" Intermittent
Line pressure and the commanded pressure agree some of the time and not the rest. On a solenoid the module modulates continuously, intermittent points at temperature and fluid condition rather than at a broken wire.
Quick facts
- System
- Powertrain
- Category
- Transmission / Pressure Control
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $120 – $1,400
- DIY difficulty
- Shop recommended
What does P0779 mean?
P0779 closes the pressure control solenoid "B" family. P0775 reports the fault generally, P0776 reports it stuck off, P0777 stuck on and P0778 electrically — and P0779 covers the case where it is none of those consistently. The correct reading of intermittent here is the thing that separates this page from its four siblings.
A pressure control solenoid is not a switch. It is a variable-force or duty-cycle-controlled valve that the transmission controller modulates continuously to set hydraulic line pressure, which in turn determines how firmly and how quickly the clutch packs apply. The controller is not asking whether the solenoid is on. It is comparing the pressure it commanded against the pressure the transmission actually produced, inferred from shift timing, ratio change rate and, on many transmissions, a direct pressure switch or transducer. So intermittent does not describe a connection that blinks. It describes agreement that comes and goes.
That points somewhere quite specific: faults that change with temperature and with the state of the fluid, rather than hard electrical faults that would set P0778 instead. A solenoid winding whose resistance drifts as it heats will control accurately from cold and inaccurately once the transmission is up to temperature. A valve bore worn oval lets the spool travel freely in thick cold fluid and stick in thin hot fluid. Debris circulating in the fluid can hold a valve off its seat for a few seconds and then wash clear. All three produce a complaint the driver describes in terms of conditions rather than events — it only does it when it's hot, only in traffic, only after an hour on the motorway.
The practical consequence is that scanning and replacing is the wrong opening move on this code. The fault is not present when the vehicle is cold on a lift, which is exactly when most diagnostics happen. What resolves it is data captured across the temperature range: a road test with fluid temperature, commanded solenoid duty and measured line pressure logged from cold through to fully warm, so the point at which command and result diverge can be seen rather than guessed.
One more thing distinguishes this member of the family. Because the fault is fluid-condition sensitive, a fluid and filter service is a genuine diagnostic step here rather than a filler recommendation. Contaminated fluid, a clogging filter and a low level all cause exactly the intermittent pressure behaviour this code describes, and they are the cheapest things on the list.
Common causes
- Low fluid level, allowing the pump to draw air intermittently under load or on corners
- Contaminated or degraded fluid carrying debris that intermittently holds a valve off its seat
- Clogged transmission filter restricting supply only once flow demand rises
- Pressure control solenoid winding resistance drifting as it heats, so control accuracy is temperature dependent
- Worn valve body bore letting the spool stick when the fluid thins at operating temperature
- Intermittent connection at the transmission case connector, where fluid and heat both attack the seal
- Chafed wiring in the internal harness between the case connector and the solenoid
- Weak or worn transmission pump unable to hold commanded pressure under sustained load
- Transmission control module output stage degrading, uncommon and diagnosed last
Symptoms
- Shifts that are firm and correct sometimes and harsh, soft or flared at other times
- The complaint correlating with temperature — normal when cold, appearing after twenty minutes of driving, or the reverse
- Occasional slipping under load that clears by itself
- Delayed engagement into drive or reverse, intermittently rather than every time
- Check engine light that comes on and later goes out on its own
- Torque converter clutch applying and releasing unevenly
- No symptom at all on a short cold test drive, which is characteristic rather than reassuring
Diagnostic steps
- 1.Check fluid level and condition first, using the manufacturer's procedure and temperature. Low, dark or burnt fluid explains intermittent pressure control on its own and is the cheapest finding available.
- 2.Read the freeze frame and note fluid temperature, vehicle speed, engine load and gear at the moment the code set. On an intermittent fault the conditions are the diagnosis.
- 3.Road test with data logging from a genuinely cold start, recording fluid temperature, commanded solenoid duty or current, and measured line pressure together. The moment command and result diverge is the fault, and it will usually correlate with a temperature threshold.
- 4.Measure solenoid resistance at the case connector cold, then again immediately after the road test while everything is hot. A reading that is in specification cold and out of specification hot is the answer, and it is invisible to a cold-only test.
- 5.Inspect the case connector for fluid intrusion, spread terminals and a hardened seal. Fluid wicking into a connector produces exactly this kind of comes-and-goes behaviour.
- 6.Wiggle-test the external harness while monitoring the solenoid circuit, but treat a negative result as inconclusive rather than as proof — the more common causes here are hydraulic, not electrical.
- 7.Fit a mechanical pressure gauge at the test port and compare actual line pressure against the commanded value across the temperature range. This separates a solenoid that is not being told correctly from a hydraulic circuit that is not responding.
- 8.If pressure tracks command accurately at all temperatures and the code still returns, look at the valve body bore and the pump rather than at the solenoid — the electrical part is doing what it was told.
Repair cost
$120 – $1,400
A fluid and filter service is $120 to $350 and is a legitimate first step on this code rather than an upsell, because contaminated fluid genuinely causes intermittent pressure control. A single pressure control solenoid is $60 to $250 for the part, but the labour dominates: most are inside the transmission behind the pan, so budget $250 to $700 all in, and considerably more on transmissions where the valve body has to come out or the unit has to be dropped. A valve body replacement runs $600 to $1,400. If line pressure is genuinely unavailable at temperature because the pump is worn, the conversation moves to an overhaul, and that should be confirmed with a gauge before it is quoted.
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.