P0235 Code: Turbocharger/Supercharger Boost Sensor A Circuit
The engine computer found an electrical fault in the boost pressure sensor's wiring or the sensor itself.
At a glance
Severity: Moderate
- Meaning
- Turbocharger/Supercharger Boost Sensor A Circuit
- System
- Fuel & Air Metering · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $0–$500
- DIY difficulty
- Moderate
The boost pressure sensor sits in the charge pipe after the intercooler or on the intake manifold and turns pressure into a voltage the engine computer can read. It runs on a 5-volt reference, a ground and a signal wire, and P0235 means the computer judged that circuit faulty as a whole: the signal dropped out, flickered or didn't behave like a working sensor, without being clearly stuck low or high. Without a trustworthy boost reading, the computer can't regulate the turbo safely, so it usually limits boost to a fixed low value and switches on the check engine light. Wiring and connector problems are at least as common as a failed sensor, and oil from the charge air creeping into the connector is a classic on turbo engines.
Can I keep driving? The engine runs, but usually with boost capped and in limp mode, so it's fine for a gentle drive to the shop. Avoid towing and full-throttle driving until the sensor circuit is repaired.
How to read P0235
- P Powertrain
- 0 Generic (SAE)
- 2 Fuel & air (injector circuit)
- 35 Specific fault
Symptoms
- Check engine light and reduced power right after start or after a bump
- Boost reading frozen, jumping or missing in live data
- Turbo feels like it never comes on
- Several unrelated sensor codes at once if a shared 5-volt reference is affected
- Problem comes and goes with engine vibration or wet weather
- Diesels: smoke under load because fuel and boost no longer match
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Oil or moisture inside the sensor connector, corroded or spread terminals
- Boost pressure sensor failed internally
- Wiring chafed against the intake, charge pipe or a bracket, open or intermittent
- Shared 5-volt reference pulled down by a short in another sensor on the same circuit
- Sensor connector not fully latched after previous work on the charge pipes
- Engine computer input fault (rare, only after wiring is proven good)
Diagnosis step by step
Check freeze frame and related codes
Note whether other 5-volt sensors, such as the throttle, rail pressure or pedal sensors, have codes too. If they do, the shared reference is the target, not the boost sensor. A lone P0235 points to this sensor's own wiring or the sensor itself.
Tools: OBD-II scanner
Watch the reading while wiggling the harness
Key on, engine off, graph the boost sensor. It should read barometric pressure, about 14.7 psi absolute at sea level, as a flat line. Gently flex the connector and the loom along its route; spikes or dropouts to zero reveal the intermittent spot.
Tools: OBD-II scanner
Inspect the connector for oil
Unplug the sensor and look inside both halves. A film of engine oil, green corrosion or a terminal that feels loose on a test pin are common findings. Oil inside often wicks through a worn sensor seal, so if you find oil, plan on replacing the sensor as well as cleaning the plug.
Tools: Flashlight, Screwdriver
Measure reference, ground and signal
Back-probe with the key on. Reference should be 4.8 to 5.2 V and ground should drop less than 0.1 V to battery negative. Signal at rest is typically 1.0 to 2.0 V on common 2.5- or 3-bar boost sensors; it should rise smoothly as boost builds, reaching roughly 3.0 to 4.0 V at full boost.
Tools: Multimeter
Do an unplug-and-jumper test
With the sensor unplugged the scan tool should show a minimum reading. Briefly jumper the signal pin to the 5-volt reference with a fused lead and it should jump to maximum. If both happen, the wiring and computer are fine and the sensor is bad.
Tools: Multimeter, OBD-II scanner
Replace and verify under boost
Fit a quality sensor with a new O-ring, clear the code and drive while graphing desired and actual boost. The actual trace should be smooth, without the notches or flat spots seen before, and follow the target closely through the rev range.
Tools: OBD-II scanner
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean and reseat the sensor connector | Yes | $0–$80 |
| Replace boost pressure sensor | Yes | $80–$320 |
| Replace sensor connector pigtail | No | $60–$220 |
| Repair chafed wiring or shorted 5-volt circuit | No | $120–$500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
A lit check engine light fails OBD-II emissions testing in states that require it. The code usually clears quickly once the circuit is repaired, but monitors still have to complete before retesting.
Frequently asked questions
What's the difference between P0235, P0236 and P0238?
P0235 is a general fault in the sensor circuit. P0236 means the signal is electrically fine but doesn't make sense against other readings, and P0238 means the signal is stuck near the top of its range. Each points to a different part of the test.
Why is there oil in my boost sensor connector?
Turbo engines carry a light oil mist in the charge air from the crankcase ventilation and turbo seals. Over time it can seep past the sensor body into the connector, where it breaks contact. Some oil in the charge pipes is normal; puddles are not and point to worn turbo seals.
Is the boost sensor the same as the MAP sensor?
Sometimes one sensor does both jobs, sometimes not. Many engines have a separate sensor before the throttle to read boost and another in the manifold. Check which one your engine uses before you buy parts.
Can I clean the sensor instead of replacing it?
Cleaning oil from the tip and the connector sometimes works if the sensor still tests good on the meter. If the reading at rest doesn't match barometric pressure or it fails the jumper test, cleaning won't bring it back.