P0303
Cylinder 3 Misfire DetectedP0303 is a generic OBD-II powertrain diagnostic trouble code: Cylinder 3 Misfire Detected. It is logged by the engine control unit when the misfire monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0303 means
P0303 is an SAE generic powertrain code that means the engine control module (ECM/PCM) has detected a misfire condition specifically in cylinder 3. The module monitors crankshaft rotational velocity in real time using the crankshaft position sensor (CKP); when cylinder 3 fails to contribute its expected power pulse, a measurable deceleration in crank speed is registered. If the misfire rate exceeds a calibrated threshold within a 200- or 1,000-revolution window, P0303 is stored and the MIL is illuminated. In severe cases, if the misfire rate is high enough to risk catalytic converter damage from unburned fuel, the MIL may flash continuously while the condition is active.
The root cause can originate in the ignition system, the fuel delivery system, or the engine mechanically. Because cylinder 3 shares the same PCM misfire-detection algorithm as every other cylinder, a systematic swap-test approach is the most reliable way to isolate which subsystem is at fault. Technicians typically swap the ignition coil and spark plug between cylinder 3 and a known-good cylinder first — if the misfire code follows the component to the new cylinder, the component is confirmed faulty. Persistent misfires after component swaps point toward a fuel injector fault or a mechanical issue such as low compression, a burnt valve, or a leaking head gasket.
Common causes
Most-frequently reported root causes when P0303 is logged.
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1
Faulty or worn spark plug in cylinder 3
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2
Defective ignition coil (or coil-on-plug pack) for cylinder 3
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3
Failed or clogged fuel injector on cylinder 3
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4
Low cylinder compression due to worn piston rings, a burnt valve, or a leaking head gasket
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5
Vacuum leak or intake manifold leak near cylinder 3
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6
Faulty crankshaft position sensor producing false misfire readings
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7
Carbon-contaminated or stuck EGR valve causing excessive exhaust gas dilution
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8
Wiring or connector damage in the cylinder 3 ignition or injector circuit
Symptoms drivers notice
How to diagnose P0303
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scanner, record all stored codes and freeze-frame data, and note the engine load and RPM at which P0303 was triggered
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2
Perform a visual inspection of the cylinder 3 spark plug, coil connector, and injector harness for obvious damage, corrosion, or oil fouling
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3
Swap the cylinder 3 spark plug and ignition coil with those from a known-good cylinder, clear codes, and road-test; if misfire migrates to the new cylinder, replace the swapped component
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4
If misfire stays on cylinder 3 after coil/plug swap, perform a fuel injector balance test or swap the injector with a known-good cylinder to rule out injector failure
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5
Perform a cylinder compression test and, if compression is within spec, a cylinder leak-down test to check for valve sealing or ring wear on cylinder 3
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6
Inspect for vacuum leaks around the intake manifold runners serving cylinder 3 using smoke or propane enrichment
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7
Check crankshaft position sensor signal quality with an oscilloscope or live data if no mechanical or ignition cause is found
Related powertrain codes
Frequently asked questions
Can I drive with a P0303 code?
Short distances at low load are generally possible, but driving with an active misfire is not recommended. Unburned fuel entering the exhaust can overheat and damage the catalytic converter, and a severe misfire can cause internal engine damage over time. If the MIL is flashing rather than steady, stop driving as soon as it is safe to do so.
Will replacing the spark plug always fix P0303?
A fouled or worn spark plug is the most common cause, but not the only one. Use the swap-test method first — move the cylinder 3 plug to another cylinder and clear the code. If the misfire follows the plug, replacement will fix it. If the misfire stays on cylinder 3, investigate the coil, injector, or engine mechanicals.
Why does P0303 sometimes appear together with P0300 (random misfire)?
P0300 is set when the PCM detects misfires distributed across multiple cylinders, or when it cannot attribute a misfire to a single cylinder. It is common to see P0300 alongside P0303 when cylinder 3 is the primary offender but the misfire is severe enough to disturb overall engine balance. Fixing the cylinder-specific fault (P0303) usually resolves P0300 at the same time.
Could a bad crankshaft position sensor cause P0303 even if cylinder 3 is healthy?
Yes. The misfire monitor relies entirely on CKP sensor signal quality. A damaged reluctor wheel, worn CKP sensor, or erratic signal can produce phantom misfire readings on any cylinder. If swapping coils, plugs, and injectors does not move the misfire and compression tests are normal, inspect the CKP sensor signal on an oscilloscope before condemning internal engine components.
Disabling P0303 in software
RaceTune can permanently disable P0303 — 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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