P02A6
Cylinder 4 - Fuel Trim at Max LimitP02A6 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 4 - Fuel Trim at Max 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 P02A6 means
P02A6 is a generic OBD-II powertrain diagnostic trouble code stored when the powertrain control module determines that the per-cylinder fuel trim correction for cylinder 4 has reached its maximum positive limit. "Maximum limit" means the PCM is adding the greatest amount of additional fuel it is permitted to add — yet cylinder 4 is still running leaner than the commanded air-fuel ratio. The PCM tracks individual cylinder combustion contribution via crankshaft acceleration signals and upstream oxygen sensor data, correlating deviations to specific cylinders. When cylinder 4 consistently under-fuels and positive trim corrections cannot bring it back to stoichiometry, P02A6 is set and the MIL illuminates. The lean condition indicated by P02A6 is more immediately dangerous to the engine than a rich condition: lean combustion raises peak cylinder temperatures, increases knock susceptibility, and risks damage to exhaust valves, piston crowns, and the catalytic converter. Common causes include a restricted or clogged fuel injector, low fuel rail pressure, an air leak introducing unmetered air into cylinder 4's intake path, or a faulty sensor providing incorrect feedback. Prompt attention is required to prevent escalation to piston or valve damage.
Common causes
Most-frequently reported root causes when P02A6 is logged.
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1
Restricted or clogged fuel injector in cylinder 4 unable to deliver the commanded fuel volume
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2
Low fuel rail pressure caused by a weak fuel pump, clogged fuel filter, or failing fuel pressure regulator
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3
Vacuum or intake air leak near cylinder 4 introducing unmetered air and leaning out the mixture
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4
Faulty or clogged fuel injector with open or shorted wiring harness reducing injector drive voltage
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5
Defective upstream oxygen sensor providing a falsely rich signal and causing the PCM to reduce fuelling
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6
Failed mass air flow sensor over-reporting air mass, causing the base fuel map to under-fuel the engine
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7
Faulty manifold absolute pressure sensor providing incorrect load data and reducing calculated injector pulse-width
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8
PCM driver channel degradation resulting in reduced electrical pulse amplitude to the cylinder 4 injector
Symptoms drivers notice
How to diagnose P02A6
A typical diagnostic flow when this code is present.
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1
Retrieve all stored DTCs and freeze frame data; note engine operating conditions (load, RPM, temperature) at the time P02A6 set
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2
Perform a cylinder 4 injector balance test using a scan tool to compare its pressure drop to adjacent cylinders — a restricted injector will show significantly less flow
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3
Test fuel pressure at the rail with a calibrated gauge and compare to manufacturer specification; low pressure must be corrected before condemning the injector
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4
Inspect for vacuum leaks at the intake manifold gasket, PCV hose, brake booster line, and any vacuum-actuated components near cylinder 4
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5
Use a DVOM to test the cylinder 4 injector connector for correct voltage and ground pulse with the engine running; absent pulse points to a wiring or PCM driver fault
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6
Check MAF sensor output in live data at idle and throttle tip-in; an over-reporting MAF will cause a system-wide lean shift that is heaviest at the cylinder farthest from the sensor
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7
Remove, ultrasonically clean, and flow-test the cylinder 4 injector if pressure and electrical checks are within specification
Related powertrain codes
Frequently asked questions
Why is the max-limit trim code (P02A6) considered more dangerous than a rich code?
A lean combustion event runs at higher peak temperature than a rich one. Sustained lean operation in cylinder 4 risks burning exhaust valves, damaging piston crowns through detonation, and overheating the catalytic converter brick. A rich cylinder wastes fuel and fouls the cat, but a lean cylinder risks structural engine damage more quickly.
What is the difference between P02A6 and P0174 (system lean bank 2)?
P0174 is a bank-level code indicating the entire bank 2 fuel trim is lean — affecting all cylinders on that bank. P02A6 is cylinder-specific, meaning only cylinder 4 is lean while the others on the same bank may be operating normally. P02A6 therefore points more directly to a cylinder 4 injector or local air leak rather than a fuel pressure or sensor issue affecting the whole bank.
Can a vacuum leak cause P02A6 on only one cylinder?
Yes. A vacuum leak specifically at the cylinder 4 intake port gasket, injector O-ring, or a runner-specific intake vacuum fitting introduces unmetered air into that cylinder alone, producing a cylinder-specific lean condition without affecting the rest of the engine.
Should I replace the injector or the oxygen sensor first?
Perform the injector balance test and fuel pressure check first — these are quick, non-invasive tests. If injector flow and fuel pressure are normal, then investigate the oxygen sensor and MAF. Replacing the injector before ruling out sensor faults or vacuum leaks can result in an unnecessary repair.
Disabling P02A6 in software
RaceTune can permanently disable P02A6 — 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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