Fuel and air

P0109 — MAP/BARO Sensor Circuit Intermittent

Severity: medium Drive with care, to a shop

What P0109 means

The engine control module constantly reads the manifold absolute pressure (MAP) sensor, and on some vehicles a barometric (BARO) sensor too. It uses this signal to estimate engine load and airflow and to set fuel delivery and ignition timing. P0109 means the signal drops out or jumps around at times in a way that does not match how the engine is running.

Symptoms

  • Check Engine light on
  • Hesitation or weak acceleration
  • Poor fuel economy
  • Rough or unstable idle
  • Intermittent loss of power
  • Occasional hard starting or stalling

Possible causes

  1. Failing or contaminated MAP/BARO sensor
  2. Poor connection or corroded terminals at the sensor connector
  3. Damaged, chafed or broken wiring, or a short to ground or voltage
  4. Intake vacuum leak or cracked vacuum hose
  5. Plugged or disconnected vacuum line to the sensor
  6. Rarely, a PCM fault or software problem

How to diagnose

  1. Scan for codes and freeze-frame data, and note any other air or fuel trim codes
  2. Inspect the sensor connector and harness for moisture, corrosion, chafing and a loose fit
  3. Wiggle the harness and connector with the engine running while watching MAP data; spikes point to a bad connection
  4. With key on, engine off, compare MAP reading to local barometric pressure; they should be close
  5. Test the sensor's 5 V reference, ground and signal wires with a multimeter
  6. Check vacuum hoses, intake gaskets and the sensor's vacuum line for leaks or blockage
  7. Watch how the signal responds to rising rpm and quick throttle changes
  8. If the wiring is good, replace the sensor and retest

Common fixes

  • Repair the wiring and clean or replace the terminals or connector
  • Replace the MAP/BARO sensor
  • Fix vacuum leaks by replacing hoses or gaskets
  • Clear or replace the sensor's vacuum line
  • Update or replace the PCM only after everything else is ruled out

Questions and answers

Can I drive with a P0109 code?

Briefly and carefully, for instance to a repair shop. The engine may lose power or stall, and a wrong mixture can harm the catalytic converter.

What most often causes P0109?

Usually a poor connection in the sensor connector or wiring, or a worn sensor. Check the connector and harness first, then replace the sensor if they are fine.

Can I just clear the code?

Clearing it may turn the light off for a while, but it will return if the fault is still there. Find the cause of the unstable signal first.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.