Fuel and air

P029B — Cylinder 1 Fuel Trim at Minimum Limit

Severity: medium Drive with care, to a shop

What P029B means

The engine control module watches the air-fuel mixture and trims injector timing for each cylinder. P029B sets when the trim for cylinder 1 has reached its lowest limit: the module is already removing as much fuel as it can, yet the mixture is still too rich. The usual culprits are that cylinder's injector or misleading sensor data.

Symptoms

  • Check Engine light on
  • Rough idle
  • Worse fuel economy
  • Reduced power and sluggish acceleration
  • Long or rough starting
  • Fuel smell or black smoke from the exhaust
  • Possible misfire on cylinder 1

Possible causes

  1. Leaking or stuck-open fuel injector on cylinder 1
  2. Faulty oxygen or air-fuel ratio sensor giving false readings
  3. Fuel pressure too high or a bad fuel pressure regulator
  4. Damaged wiring or poor connection at the injector connector
  5. Faulty mass airflow (MAF) or manifold pressure sensor
  6. Contaminated or wrong fuel
  7. ECM software issue (uncommon)

How to diagnose

  1. Read freeze frame data and all stored codes, especially misfire and oxygen sensor codes
  2. Check short-term and long-term fuel trims at idle and under load
  3. Compare trims and cylinder balance to confirm the problem is limited to cylinder 1
  4. Inspect the injector connector and wiring, and test injector resistance and control signal
  5. Measure fuel pressure and check that the system holds pressure after shutoff
  6. Pull the cylinder 1 spark plug: black sooty deposits point to a rich mixture
  7. Compare MAF and oxygen sensor readings with actual conditions, and check for air or exhaust leaks
  8. Bench-test or leak-test the injector, or have it cleaned

Common fixes

  • Clean or replace the cylinder 1 injector
  • Replace a faulty oxygen sensor or MAF sensor
  • Repair the injector wiring or connector
  • Replace the fuel pressure regulator or fuel pump if pressure is too high
  • Update the ECM software if the manufacturer offers it

Questions and answers

What does P029B mean?

The ECM found that the fuel trim for cylinder 1 has dropped to its minimum limit. That means the mixture in that cylinder is too rich and the module cannot lean it out any further.

Can I drive with P029B?

For a short time, yes, but don't put it off. A rich mixture raises fuel use and can damage the catalytic converter and spark plugs.

What most often causes P029B?

A leaking injector on cylinder 1 is the most common cause. Also check fuel pressure, the oxygen sensor and the wiring.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.