P0129

Code

Generic P — Powertrain

Barometric Pressure Too Low

Brand: Generic

Type: P — Powertrain

Views: UK: 38 EN: 85 RU: 43

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Page language: EN

Causes

Faulty barometric pressure (BARO) sensor
Open or shorted wiring between BARO sensor and ECM
Poor sensor ground or missing 5V/3.3V reference supply
Intake vacuum leak or blocked vent that affects sensor reference
ECM software or calibration error
High-altitude or rapid ambient pressure change (normal conditions may trigger code if not compensated)

Symptoms

Check Engine Light (MIL) illuminated
Reduced fuel economy or poor drivability under some conditions
Erratic idle, hesitation or stumbling
Hard starting in extreme altitude change situations
Possible altered shift behavior in automatic transmissions (if ECM uses BARO)

What to check

Read freeze frame and live BARO PID with a scan tool; note the BARO value and ambient altitude/temperature.
Compare BARO value to expected atmospheric pressure for location (sea level ≈101 kPa / 14.7 psi).
Check for other related codes (MAP, MAF, IAT) that could point to shared circuits.
Visually inspect BARO sensor, connector and wiring for damage, corrosion or disconnection.
Verify sensor supply voltage and ground with a DMM (key ON, engine OFF).
Check for intake vacuum leaks or blocked sensor vent path.

Signal parameters

Typical BARO nominal pressure near sea level: ~100–105 kPa (≈29.6–31.0 inHg) / 14.7 psi; lower at higher altitude.
Typical BARO sensor signal voltage: ~0.5–4.5 V (depends on sensor and manufacturer).
Some systems report BARO in kPa or hPa (hectopascals); expected live-data units should match scanner units.
Signal should be stable with key ON; should not drop to 0V or rail to max (indicates open/short).
BARO reading should be similar to MAP sensor reading at key ON engine OFF (MAP should read ambient pressure).

Diagnostic algorithm

Obtain freeze-frame and live data: record BARO PID and related PIDs (MAP, MAF, IAT) with a scan tool. Confirm code conditions are repeatable.
Compare BARO to known ambient pressure: use local barometric pressure (weather/altitude) or compare to MAP reading with engine off. A large discrepancy indicates fault.
Visual inspection: inspect sensor, connector and harness for damage, corrosion, moisture, or crushed wires. Repair any issues found.
Power & ground check: with key ON engine OFF, measure sensor reference voltage (5V or 3.3V per vehicle spec) and ground. Replace/repair wiring if reference or ground missing.
Signal test: backprobe signal wire and observe voltage with key ON and during engine start. Voltage should be within expected range and change with altitude or when covering/uncovering vent (if accessible).
Compare BARO to MAP: with engine OFF, BARO and MAP should read nearly identical ambient pressure. If MAP reads correctly and BARO does not, suspect BARO sensor.

Likely causes

Damaged or failed BARO sensor
Connector corrosion or loose connector at the sensor
Broken wire, chafed harness, or intermittent connection on signal, power or ground
ECM input circuit fault
Blocked sensor vent or contamination causing incorrect reading
Vehicle recently moved to much higher altitude without ECM recalibration

Fault status

⚠️ Status

P0129 — Barometric Pressure Too Low: stored when the ECM detects BARO sensor readings below expected thresholds or inconsistent with other pressure inputs. May be caused by sensor, wiring, or actual low ambient pressure and can affect fuel trims and emissions control.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2 hours