P0129: Barometric Pressure Too Low

What it means

Barometric pressure reading below the plausible minimum. Either operating at extreme altitude or sensor fault.

At a glance

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

How it works

The barometric pressure sensor provides the module with an estimate of atmospheric pressure so it can adjust engine operation for altitude and air density. P0129 sets when the reported pressure is below the plausible range, which can occur at extreme altitude or when the sensor signal or circuit is faulty.

Common causes

Most likely first.

1. Barometric pressure sensor reading inaccurately low
2. Sensor wiring or connector fault
3. Engine control module fault

Symptoms

• Check engine light on
• Reduced engine power
• Sluggish acceleration

How to diagnose it

1. Review freeze-frame data and compare the barometric pressure reading with local altitude and other available pressure readings.
2. Check whether the vehicle is operating at unusually high altitude, where lower atmospheric pressure may be expected.
3. Inspect the barometric pressure sensor connector and wiring for corrosion, damage, loose terminals, or poor contact.
4. Compare the sensor reading with manifold pressure data under appropriate conditions to identify an implausible discrepancy.
5. Test the sensor supply, ground, signal, and sensor response against the manufacturer's specification.

Common fixes

• Clean and secure a loose or corroded sensor connector.
• Repair damaged wiring or a poor connection in the barometric pressure circuit.
• Replace the barometric pressure sensor if it reads inaccurately during testing.
• Investigate a module fault only after the sensor and its circuit pass checks.

Can I keep driving?

Reduced power or sluggish acceleration may make merging and overtaking more difficult, so drive cautiously and avoid demanding acceleration. If power loss becomes severe, stop somewhere safe and arrange diagnosis.