P029B
Cylinder 1 - Fuel Trim at Min LimitP029B is a generic OBD-II powertrain diagnostic trouble code: Cylinder 1 - Fuel Trim at Min Limit. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P029B means
P029B is stored when the powertrain control module detects that the per-cylinder fuel trim correction for cylinder 1 has reached or exceeded the maximum negative limit — meaning the PCM has had to reduce fuel delivery to cylinder 1 as far as the calibration allows in an attempt to correct an over-rich condition on that cylinder. Per-cylinder fuel trim operates by independently adjusting injected quantity for each cylinder based on crankshaft acceleration feedback and oxygen sensor data to balance combustion output across the engine. When cylinder 1 consistently produces more torque than the commanded amount — indicating over-fuelling — the PCM reduces the injected quantity until the correction hits the negative minimum, at which point the code sets and the MIL is illuminated. On common-rail diesel and GDI engines, the most significant mechanical cause of cylinder 1 over-fuelling is a leaking or stuck-open fuel injector: a needle that does not seat cleanly delivers excess fuel into the cylinder beyond the commanded injection event. On diesel engines this leads to rich combustion, elevated exhaust smoke, and potential fuel dilution of the engine oil — a dangerous condition because diesel injector leak-down past the needle seal contaminates the sump. On GDI petrol engines a leaking high-pressure injector causes rich misfire, hydrocarbon emissions, and a characteristic raw fuel smell. The condition is the functional mirror image of P029A: where P029A describes maximum positive trim (under-fuelling corrected upward), P029B describes minimum negative trim (over-fuelling corrected downward). Both reflect the PCM's inability to bring cylinder 1 combustion into balance within its normal adjustment range.
Common causes
Most-frequently reported root causes when P029B is logged.
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1
Leaking or stuck-open cylinder 1 fuel injector delivering excess fuel beyond the commanded injection quantity — the primary cause on diesel and GDI engines
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2
Injector needle not seating fully due to coking on the needle seat land or mechanical wear allowing continuous dribble fuelling between injection events
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3
High fuel rail pressure beyond specification (faulty pressure regulator or pressure relief valve) forcing excess fuel through the cylinder 1 injector
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4
Crankshaft acceleration or combustion contribution sensor error reporting artificially low torque from cylinder 1, causing the PCM to misread a normal cylinder as over-fuelling
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5
Faulty oxygen sensor providing a persistently rich signal for the exhaust sector covering cylinder 1, driving the PCM to trim fuel down to the negative minimum
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6
MAF or MAP sensor over-reading air mass, causing the PCM to underfuel all cylinders globally while cylinder 1's individual trim independently reaches minimum due to a concurrent injector issue
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7
ECM software or calibration fault causing incorrect per-cylinder trim boundaries for cylinder 1 (rare; check for available ECM updates before replacing hardware)
Symptoms drivers notice
How to diagnose P029B
A typical diagnostic flow when this code is present.
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1
Connect a scan tool, log all DTCs and freeze-frame data, and check for companion codes — P0301, P0172/P0175, or rail pressure codes; a leaking injector often produces multiple fault codes simultaneously
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2
Check engine oil dipstick for fuel dilution — if oil level is elevated or smells of diesel/petrol, a leaking injector has been contaminating the sump; change the oil before running the engine further
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3
Use live scan data to monitor per-cylinder contribution values at idle; cylinder 1 showing a higher than average torque contribution confirms over-fuelling on that cylinder
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4
Perform an injector leak-down or return-flow test with the engine stopped — measure how much the cylinder 1 injector drains over a fixed time period without energising; a result above the OEM specification confirms a leaking needle seat
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5
Verify high-pressure fuel rail pressure; a faulty pressure regulator driving rail pressure above specification increases injection quantity for all injectors, but a worn needle on cylinder 1 will leak disproportionately at elevated rail pressure
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6
Inspect the cylinder 1 injector connector and wiring for a short to voltage that could be holding the solenoid partially energised, causing the needle to remain cracked open
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7
Replace the cylinder 1 injector (and recode IMA/IQA/C2I values for diesel platforms), change the engine oil if dilution is confirmed, clear all DTCs, and complete a full drive cycle to verify the per-cylinder trim returns to within normal range
Related powertrain codes
Frequently asked questions
Is a leaking injector dangerous enough to warrant immediate shutdown?
Yes, if fuel dilution of the engine oil is confirmed. Diesel or petrol mixing into engine oil reduces its viscosity and lubrication effectiveness, risking bearing damage during extended operation. If the oil level is rising or smells strongly of fuel, the vehicle should not be driven until the injector is replaced and the oil changed. Hydraulic lock from injector drip-down is also a risk if the vehicle sits unused with a leaking injector.
Can P029B appear on a port-injected petrol engine?
It is uncommon on port-injection engines because most lack per-cylinder individual trim capability — they use bank-level trims only. P029B is most prevalent on common-rail diesel engines and gasoline direct injection (GDI) engines where the PCM has enough resolution to track individual cylinder fuelling balance.
What is the difference between P029B and a global rich code like P0172?
P0172 means the bank-wide fuel trim has reached its negative minimum — the entire bank is over-fuelling. P029B is localised: only cylinder 1's individual trim has hit the minimum limit, while other cylinders may be fuelling correctly. P029B therefore points directly to a cylinder 1 specific fault such as a leaking injector rather than a systemic issue.
How do I confirm an injector is leaking before replacing it?
The most reliable method is a return-flow (back-leak) test: measure injector return flow volume over a set period without energising the injectors. An excessively high return from cylinder 1's injector confirms the needle is not seating fully. A stethoscope can also detect a continuous dripping sound from a leaking injector with the engine off and rail pressure bled down, and a combustion pressure gauge or balance test can show cylinder 1 over-contributing at idle.
Disabling P029B in software
RaceTune can permanently disable P029B — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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