P0129: Barometric Pressure Too Low

What does P0129 mean?

The engine control module needs the ambient air pressure for mixture, altitude correction and boost limits. It comes either from a dedicated barometric sensor or from the MAP sensor, read at key-on with the engine stopped and updated at wide-open throttle. P0129 means this value is lower than physically possible – even on the highest drivable passes air pressure stays around 50 kPa, and GM and Ford use 50 kPa as their lower plausibility limit. Usual causes are a MAP or baro sensor reading too low, a blocked sensor port or high resistance in the wiring. No manufacturer document naming exactly this code was found: GM and Ford file the same finding under P2227.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light on
• Rich mixture, higher consumption, rough idle
• Loss of power (altitude-corrected boost limits)

Common causes

• High MAP or baro sensor reading too low (dirty, oiled, drifted)
• Medium High resistance or corrosion in signal wire or connector
• Medium Sensor port or hose blocked
• Low Control module (baro sensor often built in) or software

Likelihood is a general estimate across vehicles; your make and model may differ.