Faulty barometric pressure sensor or MAP sensor reading low or drifting; Damaged wiring, poor connector contact or corrosion in the sensor circuit (signal, reference voltage or earth); Blocked, kinked, cracked or disconnected vacuum hose or sensor port, where the sensor reads through a hose; Restricted air intake, such as a blocked air filter or collapsed intake duct, causing genuinely low pressure readings; Intake leak or other fault affecting the MAP reading at key-on or during the plausibility check; Fault in the engine control unit.

P0129

Barometric Pressure Too Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
1: AreaFuel and air metering
29: FaultThe specific fault in that area

What it means

P0129 is the generic code for barometric pressure too low. The engine control unit has judged that the barometric pressure reading, from a dedicated sensor or one built into another sensor such as the MAP sensor, is lower than expected for the conditions. It usually sets when the reading is implausibly low compared with other inputs such as MAP at key-on, throttle position or altitude. How the reading is obtained and checked varies by model and engine.

Symptoms

Engine warning light on
Poor performance or hesitation under load
Rough idle or unstable idle speed
Poor fuel economy
Hard starting or stalling in some cases
Engine running rich or lean due to incorrect fuel and air calculations

Possible causes

Faulty barometric pressure sensor or MAP sensor reading low or driftinglikely
Damaged wiring, poor connector contact or corrosion in the sensor circuit (signal, reference voltage or earth)likely
Blocked, kinked, cracked or disconnected vacuum hose or sensor port, where the sensor reads through a hosepossible
Restricted air intake, such as a blocked air filter or collapsed intake duct, causing genuinely low pressure readingspossible
Intake leak or other fault affecting the MAP reading at key-on or during the plausibility checkpossible
Fault in the engine control unitrare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting load, engine speed and conditions when the code set, and check for other sensor codes that point to a shared cause
Visually inspect the sensor, its connector, wiring and any vacuum hose for damage, chafing, corrosion, water ingress or splits
Check the air filter, intake ducting and airbox for blockages or collapse
With the ignition on and engine off, compare the live barometric or MAP reading with the known local atmospheric pressure; it should be close to it
Check the live reading during a drive or throttle snap and watch for implausible dips, dropouts or readings stuck low
Check the sensor supply voltage, signal and earth at the connector against the manufacturer's specification for that model
Check for intake leaks and vacuum hose condition if the sensor is connected by a hose
Wiggle-test the wiring and connector while watching live data to find intermittent faults

How to fix

Replace the barometric pressure or MAP sensor if it reads incorrectly and the wiring is sound
Repair or replace damaged wiring and connectors, and clean or refit corroded terminals
Replace or reconnect any cracked, blocked or loose vacuum hose to the sensor
Replace a clogged air filter and clear any intake restriction
Repair any intake leak found, then clear the code and road test to confirm it does not return
Check for a manufacturer software update only if a known calibration fault applies to that model, and diagnose the control unit fault only after everything else has been ruled out

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but fuel and air calculations may be wrong, so performance and economy can suffer. Book it in soon, and avoid heavy loads or long journeys if it runs badly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.