What P0109 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 1, places it in the fuel and air metering subsystem; the last two digits identify the specific fault.
The MAP sensor measures pressure inside the intake manifold, which tells the engine computer how hard the engine is working. On speed-density engines it is the main load input; on engines with a MAF it is used as a cross-check. Before start-up, and on some engines at wide-open throttle, the same reading stands in for barometric pressure.
What “Intermittent” means here
"Intermittent" (or "Erratic") means the manifold absolute pressure (MAP) / barometric pressure sensor signal dropped out or jumped in a way a working sensor could not, but not continuously. Intermittent faults are almost always connections: a loose or corroded terminal, a chafed harness, or a wire that breaks contact with vibration or heat. Watching live data while gently flexing the harness is the classic way to catch one.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0109:
- Rough idle or hesitation
- Black smoke or a fuel smell if the engine runs rich
- Poor fuel economy
- Hard starting on some engines
Where to start diagnosing P0109
- Inspect the vacuum hose or port feeding the sensor for cracks, blockage or oil contamination.
- Check the connector and the 5 V reference and ground at the sensor.
- With the key on and engine off, the MAP reading should roughly equal local barometric pressure; at a warm idle it should drop well below it.
- Look for vacuum leaks, which raise manifold pressure at idle.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0109. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Intake manifold absolute pressure (PID 0x0B) | Key-on engine-off should match barometric pressure; idle should be markedly lower. |
| Barometric pressure (PID 0x33) | Should match local conditions for your altitude. |
| Engine RPM (PID 0x0C) | MAP should fall as the throttle snaps shut from high rpm. |
Related codes
Codes from the same manifold absolute pressure (MAP) / barometric pressure sensor family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
| Code | Definition |
|---|---|
| P0105 | Manifold Absolute Pressure/Barometric Pressure Circuit |
| P0106 | Manifold Absolute Pressure (MAP) Sensor Circuit Range/Performance |
| P0107 | Manifold Absolute Pressure/Barometric Pressure Circuit Low |
| P0108 | Manifold Absolute Pressure/Barometric Pressure Circuit High |
| P0129 | Barometric Pressure Too Low |
Further reading
Frequently Asked Questions
What does P0109 mean?
P0109 is the generic SAE J2012 code for "Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent/Erratic". In plain terms, it means the engine computer has detected a "intermittent" fault on the manifold absolute pressure (MAP) / barometric pressure sensor.
Can I keep driving with P0109?
Drivability varies with how much the engine relies on the MAP signal. If the engine stumbles or runs rich, limit driving until it is checked.
What should I check first for P0109?
Inspect the vacuum hose or port feeding the sensor for cracks, blockage or oil contamination.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.