Leaking or stuck-open fuel injector on cylinder 1 delivering excess fuel; Contaminated or failed oxygen sensor on bank 1 reading falsely lean, driving the computer to add fuel; Faulty engine coolant temperature sensor reporting a cold engine so the computer commands a rich mixture; Leaking intake manifold gasket or injector O-ring admitting unmetered air that skews fuel trim learning; Restricted or dirty mass airflow sensor element causing inaccurate air measurement; Worn spark plug or ignition coil on cylinder 1 causing incomplete combustion and a false rich reading; Leaking evaporative emissions purge valve allowing fuel vapor into the intake when it should be closed

DTC codes / P02xx · Fuel and air metering (injector circuits) / P029B

P029BCylinder 1 - Fuel Trim at Min Limit

Q: What does code P029B mean?

Cylinder 1 - Fuel Trim at Min Limit. P029B means the engine computer has detected that the fuel trim correction for cylinder 1 has reached its minimum limit, so the powertrain control module can no longer remove enough fuel to bring that cylinder's mixture back to stoichiometric. Cylinder 1 is running so rich that the computer has maxed out its ability to lean it out, which points to excess fuel entering that cylinder or a false rich signal from its oxygen sensor.

P029B means the engine computer has detected that the fuel trim correction for cylinder 1 has reached its minimum limit, so the powertrain control module can no longer remove enough fuel to bring that cylinder's mixture back to stoichiometric. Cylinder 1 is running so rich that the computer has maxed out its ability to lean it out, which points to excess fuel entering that cylinder or a false rich signal from its oxygen sensor.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaFuel and air metering (injector circuits)

Symptoms you may notice

Illuminated check engine light, often with no other symptoms besides the warning light
Rough idle or a noticeable shake that is worse when the engine is cold
Black sooty exhaust smoke and a strong fuel smell at the tailpipe
Poor fuel economy and a gradual loss of engine power
Misfire-like shudder under acceleration if the excess fuel fouls the spark plug
Elevated hydrocarbon readings or a failed emissions test

Common causes, most likely first

Leaking or stuck-open fuel injector on cylinder 1 delivering excess fuel
Contaminated or failed oxygen sensor on bank 1 reading falsely lean, driving the computer to add fuel
Faulty engine coolant temperature sensor reporting a cold engine so the computer commands a rich mixture
Leaking intake manifold gasket or injector O-ring admitting unmetered air that skews fuel trim learning
Restricted or dirty mass airflow sensor element causing inaccurate air measurement
Worn spark plug or ignition coil on cylinder 1 causing incomplete combustion and a false rich reading
Leaking evaporative emissions purge valve allowing fuel vapor into the intake when it should be closed

How to diagnose and fix it

Scan all modules and record freeze-frame data for P029B, then check for related misfire or oxygen sensor codes before touching any parts
Inspect the spark plug on cylinder 1 for black soot or fuel fouling and compare it with the other cylinders
Measure long-term and short-term fuel trim on bank 1 at idle and at 2500 rpm to see whether the rich condition is cylinder-specific or bank-wide
Test the cylinder 1 fuel injector for leakage, resistance and spray pattern, and swap it with a known-good injector to see if the fault follows
Check the engine coolant temperature sensor reading against actual engine temperature with a scan tool or infrared thermometer
Smoke-test the intake system for vacuum leaks and inspect the evaporative purge valve for sticking open
Verify the bank 1 upstream oxygen sensor responds correctly to a propane enrichment test before condemning it

Why cylinder 1 fuel trim maxes out

A rich fuel trim limit on a single cylinder is often caused by a leaking injector rather than a sensor fault, so swapping the injector to another cylinder is the fastest way to confirm it.
A lazy upstream oxygen sensor can report a false lean condition that makes the computer add fuel until the trim hits its minimum, even though the actual mixture is correct.
Technicians frequently replace oxygen sensors for this code when the real cause is a sticking purge valve or a leaking injector O-ring that lets raw fuel into the intake.
Fuel trim limits are learned values, so after a repair the code may return until the adaptive memory is cleared and a relearn drive cycle is completed.
A misfire on cylinder 1 from a worn plug or coil can push unburned fuel into the exhaust, which the oxygen sensor reads as rich and drives the trim to its limit.

Can I keep driving with P029B?

You can usually drive the vehicle to a workshop, but avoid hard acceleration and long trips because excess fuel can wash cylinder walls, foul the spark plug and overheat the catalytic converter. If the check engine light starts flashing or the engine misfires badly, stop driving and have it towed.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.