P0109: Manifold Absolute Pressure / Barometric Pressure Circuit Intermittent

What it means

The ECU detects intermittent signal dropouts on the MAP/BARO sensor circuit. Fault is not constantly present — typically caused by loose connectors, chafed wiring, or a sensor failing under thermal load.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The MAP sensor measures pressure in the intake manifold, and some vehicles also use a barometric pressure signal. The engine control module monitors this circuit for a stable, plausible signal; P0109 sets when the MAP/BARO reading drops out or changes erratically at times rather than staying continuously faulty.

Common causes

Most likely first.

1. MAP connector loose or corroded
2. Sensor fails when hot
3. Wiring chafed or pinched

Symptoms

• Check engine light on

How to diagnose it

1. Check freeze-frame data and watch MAP/BARO live data to see whether the reading drops out, especially around the conditions recorded when the code set.
2. Inspect the MAP sensor connector for looseness, corrosion, or damaged terminals that could interrupt contact.
3. Check the sensor wiring for chafing, pinching, or heat damage, and gently move the harness while monitoring the signal for dropouts.
4. If the fault appears after warm-up, compare cold and hot live readings to see whether the sensor fails as it heats.
5. Test the MAP/BARO circuit and sensor against the manufacturer's specification, including continuity and power or ground where applicable.

Common fixes

• Reconnect and secure a loose MAP sensor connector.
• Clean or repair corroded connector terminals.
• Repair chafed or pinched wiring and secure the harness away from heat or moving parts.
• Replace the MAP/BARO sensor if it intermittently fails testing, particularly when hot.

Can I keep driving?

With only the check-engine light on, short, gentle trips may be possible if the engine runs normally. Avoid hard acceleration if it starts running poorly; an intermittent pressure signal can disrupt fueling and increase emissions.