P0266

Cylinder 2 Contribution/Balance Fault

P0266 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 2 Contribution/Balance Fault. It is logged by the engine control unit when the fuel/inj monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0266
Group
Powertrain
System
Fuel/Inj
Severity
Warning (MIL on, possible limp mode)
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What P0266 means

P0266 is set when the ECM's cylinder-balance monitor determines that cylinder 2 is not contributing its expected share of engine torque. The ECM infers each cylinder's individual contribution by measuring crankshaft acceleration during the power stroke of each cylinder using the crankshaft position sensor; a cylinder that produces significantly less acceleration than the average of its peers triggers this fault.

The balance monitor is found on virtually all modern diesel engines — where precise per-cylinder fuelling via high-pressure common-rail injectors is critical — and increasingly on direct-injection gasoline platforms such as Ford EcoBoost/GTDI, GM GDI, and Volkswagen TFSI engines. On diesels the usual culprit is a clogged, stuck-open, or leaking injector or a drop in compression from worn rings or a leaking head gasket. On gasoline engines, a fouled or weak fuel injector, a failing ignition coil, or a worn spark plug are the common causes.

P0266 is often accompanied by a companion misfire code (P0302) and may produce noticeable rough running and power loss. The fault should be diagnosed promptly: a cylinder that consistently under-contributes can cause the ECM to over-fuel neighbours in compensation, increasing emissions and potentially damaging the catalytic converter or DPF.

Common causes

Most-frequently reported root causes when P0266 is logged.

  • 1
    Clogged or partially blocked cylinder 2 fuel injector delivering insufficient fuel mass per injection event.
  • 2
    Leaking or stuck-open injector on cylinder 2 that dribbles fuel rather than producing a clean spray pattern.
  • 3
    Weak or failed ignition coil on cylinder 2 (gasoline engines) causing incomplete combustion.
  • 4
    Worn or fouled spark plug on cylinder 2 (gasoline engines) that misfires under compression.
  • 5
    Low cylinder compression from worn piston rings, a leaking valve, or a blown head gasket on cylinder 2.
  • 6
    Corroded or damaged wiring to the cylinder 2 injector causing intermittent under-fuelling.
  • 7
    Inaccurate crankshaft or camshaft position sensor signal producing false cylinder-balance readings.

Symptoms drivers notice

MIL illuminated with P0266 stored, often alongside a cylinder 2 misfire code (P0302).
Rough idle with an irregular RPM fluctuation, most pronounced when the engine is cold.
Hesitation or flat spot during acceleration, particularly at low to mid engine speeds.
Reduced overall engine power and slower throttle response under load.
Elevated fuel consumption or abnormal long-term fuel trim as the ECM compensates for cylinder 2 shortfall.
On diesel engines, possible black or grey exhaust smoke from incomplete combustion on cylinder 2.

How to diagnose P0266

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all codes and freeze-frame data with a scan tool; note whether a concurrent P0302 misfire and any fuel-trim or boost-related codes are present.
  2. 2
    Perform a live-data cylinder-balance test (available on most professional scan tools for diesel and GDI petrol engines): disable each injector in turn and compare the RPM drop — a small drop when cylinder 2 is disabled confirms low contribution.
  3. 3
    Inspect and test the cylinder 2 fuel injector: check electrical resistance, observe the spray pattern if possible, and on common-rail diesels compare injector return-flow (leak-off) against specification.
  4. 4
    On gasoline engines, inspect the cylinder 2 spark plug for fouling, erosion, or damage and test the coil's primary and secondary resistance; swap the coil to another cylinder to confirm if the fault follows.
  5. 5
    Perform a compression test and/or a leak-down test on cylinder 2 to rule out mechanical causes such as worn rings or a leaking valve.
  6. 6
    Check fuel pressure at the rail and individual injector balance rates from live data to rule out a global low-fuel-pressure condition affecting only cylinder 2 disproportionately.
  7. 7
    After repairs, clear codes and run a full drive cycle with cylinder-balance monitoring active to confirm P0266 does not return.

Related powertrain codes

Frequently asked questions

Is P0266 a diesel-only code?

No. While the cylinder-balance monitor originated on diesel engines and is especially sensitive there — because diesel ECMs actively trim per-injector fuel quantity — modern direct-injection gasoline platforms (Ford EcoBoost, GM GDI, VW/Audi TFSI) use the same crankshaft-acceleration method and can set P0266 for spark, coil, or injector faults on cylinder 2.

Why does P0266 often appear together with P0302?

Both monitors observe the same cylinder 2 power-stroke event. P0302 is the misfire monitor (counts crankshaft deceleration events per 200 or 1000 revolutions), while P0266 is the contribution/balance monitor (compares each cylinder's acceleration against peers). A cylinder with poor fuelling or ignition will trigger both thresholds, so both codes are stored simultaneously.

Can a dirty injector cause P0266, and will cleaning fix it?

Yes, deposit build-up is one of the most common causes, particularly on high-pressure GDI and common-rail diesel injectors. On-car ultrasonic cleaning or a professional injector flow-test and clean can restore correct spray pattern and flow volume. However, if the injector nozzle is worn or the solenoid/piezo actuator is degraded, cleaning will not be sufficient and replacement is required.

How is low compression distinguished from an injector fault as the cause of P0266?

A compression test is the definitive check: low compression on cylinder 2 (typically more than 10–15% below the other cylinders or below the OEM minimum) points to a mechanical cause such as worn rings, a bent valve, or a head gasket leak. A follow-up leak-down test identifies whether the loss is past the rings, past the valves, or into the coolant. If compression is normal, focus diagnostic attention on the injector and ignition system.

Disabling P0266 in software

RaceTune can permanently disable P0266 — 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.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

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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