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DTC codes / P02xx · Fuel and air metering (injector circuits) / P02A6

P02A6Cylinder 4 - Fuel Trim at Max Limit

P02A6 means the engine computer has detected that cylinder 4's fuel trim has reached its maximum correction limit, indicating the engine is running excessively lean on that cylinder and the computer can no longer add fuel to compensate. The powertrain control module sets this code when the long-term fuel trim for cylinder 4 stays at or near its richest command for a calibrated time, often because of a vacuum leak, weak fuel delivery, or a faulty injector.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering (injector circuits)

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • Rough idle or misfire that is more pronounced when the engine is cold
  • Hesitation or stumble during acceleration, especially under load
  • Reduced fuel economy despite the computer trying to add fuel
  • Occasional surging or uneven power delivery from the affected cylinder
  • A faint fuel odor or black smoke from the tailpipe in severe cases

Common causes, most likely first

  1. Vacuum leak at the intake manifold gasket, throttle body, or a cracked vacuum hose feeding cylinder 4
  2. Clogged or failing fuel injector on cylinder 4, restricting fuel flow
  3. Weak fuel pump or restricted fuel filter causing low fuel pressure across the rail
  4. Dirty or contaminated mass airflow sensor reporting less air than actually enters the engine
  5. Leaking intake manifold runner gasket or cracked intake manifold specific to cylinder 4
  6. Exhaust leak upstream of the oxygen sensor fooling the computer into adding fuel
  7. Mechanical issue such as a worn camshaft lobe or valve seat causing poor cylinder breathing

How to diagnose and fix it

  1. Connect a scan tool and record freeze-frame data plus long-term and short-term fuel trim values for all cylinders to confirm the lean condition is isolated to cylinder 4
  2. Perform a visual inspection of intake hoses, vacuum lines, and the intake manifold gasket for cracks or loose connections, using a smoke machine if available
  3. Check fuel pressure at the rail with a gauge and compare to the manufacturer's specification, replacing the filter if pressure is low
  4. Clean or swap the cylinder 4 fuel injector with a known-good unit to see if the trim changes, then replace the injector if the fault follows it
  5. Clean the mass airflow sensor with approved cleaner and verify its readings against expected values at idle and under load
  6. Run a cylinder contribution or compression test to rule out mechanical faults such as a bad valve or worn piston rings
  7. After repairs, clear the code and drive the vehicle through a complete readiness cycle to confirm the fuel trim returns to normal and the code does not reset

Why cylinder 4 fuel trim maxes out

  • A vacuum leak at the intake manifold gasket often affects only one cylinder on engines with individual runner gaskets, so a smoke test focused on cylinder 4's runner is more revealing than a general engine bay smoke test.
  • Fuel trim at max limit does not always mean the injector is bad; a weak fuel pump can cause the same code on one cylinder first because that injector may be the most sensitive to pressure drops.
  • Technicians sometimes replace the oxygen sensor when they see a lean code, but the sensor is usually reporting correctly and the real fault is unmetered air or insufficient fuel delivery.
  • Exhaust leaks before the oxygen sensor can pull in outside air and cause the computer to add fuel, mimicking a lean condition on a single cylinder if the leak is near that cylinder's exhaust port.
  • On engines with variable valve timing, a stuck phaser or oil control valve can alter cylinder 4's breathing and set a fuel trim code without any obvious misfire.

Can I keep driving with P02A6?

You can usually drive the vehicle short distances to a repair shop, but avoid hard acceleration and heavy loads because the lean condition on cylinder 4 can cause misfires and overheating that may damage the catalytic converter. If the check engine light starts flashing, stop driving and have the vehicle 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.

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