P0109
PowertrainManifold Absolute Pressure/Barometric Pressure Circuit Intermittent
The ECU watches the voltage coming back from the manifold absolute pressure sensor, and on a turbo or supercharged engine that reading tells it exactly how much air is going in so it can match fuel and timing. With P0109, that signal isn't holding steady. It drops out or jumps about briefly, then settles, which is why the code says 'intermittent'. For you that means the car runs fine most of the time then occasionally hesitates or feels flat before sorting itself out, and the fault is usually a flaky connection rather than a dead sensor.
ⓘ Information only. This page provides general educational information about fault code P0109. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0109 mean?
P0109 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light, sometimes coming and going rather than staying on solid
- • Idle that goes a bit lumpy for a few seconds then steadies on its own
- • A momentary stumble or flat spot pulling away, often gone before you've thought about it
- • Brief power dip, the sort that makes you check your mirrors then forget it happened
- • Slightly heavier fuel use over a tank, nothing dramatic
- • Occasional fussy starting, usually warm rather than cold
Possible causes
Causes commonly associated with P0109, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Corroded or loose pins in the MAP sensor connector, by far the most common reason for an intermittent code. The contact opens up over a bump or with heat then closes again, which is exactly what the ECU is flagging
- 2. A poor earth or chafed wire in the sensor harness, often where the loom rubs near the manifold and the heat has dried out the insulation
- 3. The MAP sensor itself going lazy, frequently because the sensing port is gummed up with oil mist on a turbo engine that's been through a few sets of intake hoses
- 4. A small vacuum or boost leak letting unmetered air past, which makes the pressure reading wander
- 5. Out of date ECM software on certain models where the manufacturer has issued an update for noisy sensor signals
- 6. An internal ECM fault affecting how it reads the sensor voltage, rare and the last thing to suspect
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Wiggle the MAP connector and the harness near it while watching live data, ideally with the engine warm. If the reading glitches when you move things, you've found your intermittent and it's almost always in there
- 2. Pull the connector and look hard at the pins for green corrosion, spread terminals, or oil contamination, then clean them up and reseat. Cheap fix and it cures a fair share of these
- 3. Read live MAP data against engine load and compare it to barometric pressure at key-on. At idle it should sit low, around 1 volt or so depending on the system, and climb smoothly under boost with no spikes
- 4. Check for any other stored codes. P0107 or P0108 sitting alongside points more firmly at the sensor or wiring, lean codes like P0171 hint at a leak instead
- 5. Smoke test the intake and boost pipes to rule out a leak, paying attention to clamps and the joints in the turbo plumbing where they work loose
- 6. If wiring, sensor and intake all check out, look up any TSB for your model and only then consider the ECM. Don't go near reprogramming until everything cheaper is eliminated
Common questions about P0109
Will my car fail the MOT with a P0109 stored? +
The code on its own isn't an MOT failure item. What the tester sees is the dashboard. If the engine warning light is on when the car goes in, that's a fail under the current rules. Because P0109 is intermittent, the light may be off on the day, in which case you'll get through, but that's luck rather than a fix. Sort the cause and clear the light properly before booking it in.
What's this likely to cost me to put right? +
A connector clean and re-pin is more or less free if you do it yourself, or maybe £40 to £80 of diagnostic time at an independent garage. A MAP sensor runs roughly £30 to £120 for the part on most turbo engines, plus a bit of labour. An independent will usually have you sorted for under £150 if it's the sensor or wiring. If it turns into ECM reprogramming or a replacement module at a main dealer, you're into several hundred pounds, sometimes four figures, which is exactly why you exhaust the cheap checks first.
How do I tell which of these is actually wrong on my car? +
Start with the wiggle test on the connector and harness while watching live data. If the reading jumps when you disturb it, the fault is electrical and you've narrowed it right down. If the sensor reads steady but the figures look off compared to expected boost, suspect the sensor or a leak, and a smoke test separates those two. Lean fuel-trim codes alongside it lean toward a vacuum leak. Clean signal, no leak, and it keeps returning after the wiring is sound is the only point at which the ECM comes into the conversation.
Can I just clean it or swap the sensor myself? +
Yes, and that's the sensible first move. Unplug the MAP connector, give the pins a clean with electrical contact cleaner, check none are pushed back or corroded, and reseat it firmly. That alone fixes a lot of intermittent codes. If you want to replace the sensor it's usually one or two bolts and a plug, ten minutes of work. The part you can't fix at home is a properly intermittent wiring fault buried in the loom, and you won't fix the ECM with a spanner.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →