P0109: the manifold pressure sensor circuit cuts in and out
Some engines measure engine load with a manifold pressure sensor instead of, or alongside, an airflow sensor: it reads the pressure inside the intake manifold, which changes with how far the throttle is open and how hard the engine is working. This code means that sensor's circuit signal comes and goes, or briefly drops out and returns, rather than failing steadily in one direction. It's an intermittent fault, which can make it harder to catch on a single check because it isn't present all the time.
You might notice
- The engine warning light comes on
- The engine may hesitate or run unevenly, on and off rather than all the time
- manifold pressure sensor
- — a sensor that measures the pressure inside the intake manifold, which the computer uses to judge engine load
What the code itself says
P0109 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
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