What is a po303 code on a Lexus?
In Lexus vehicles, P0303 indicates a cylinder 3 misfire. The engine control module detects that cylinder 3 is not firing correctly, causing rough running, reduced power, and higher emissions. Note: the correct code is P0303 (not PO303).
Understanding P0303
What the code means
P0303 is a cylinder-specific misfire diagnostic code. It means the engine’s computer has detected that the third cylinder is not firing as it should during normal operation.
Why it’s tracked by the system
The misfire detection system uses data from the crankshaft sensor, ignition system, and fuel delivery to determine if a cylinder fails to ignite consistently. A sustained misfire can trigger the check engine light and set P0303 in the Lexus’ onboard computer.
Common Causes
The main categories of causes for a cylinder 3 misfire on a Lexus include issues with ignition, fuel delivery, and engine integrity.
- Ignition problems: worn or fouled spark plug in cylinder 3, faulty ignition coil for cylinder 3, damaged spark plug boot, or degraded coil wiring.
- Fuel delivery problems: malfunctioning or dirty fuel injector for cylinder 3, inadequate fuel pressure, or a clogged fuel filter.
- Air and vacuum leaks: intake manifold vacuum leaks or leaks around cylinder 3 can disrupt the air-fuel mix.
- Mechanical issues: low compression in cylinder 3 due to worn rings, a valve issue, or timing problems.
- Electrical or sensor issues: faulty cam/crank position sensor data or wiring faults affecting cylinder 3 components.
Understanding these categories helps prioritize diagnostics and repairs.
Symptoms to Expect
When P0303 is active, drivers commonly notice several telltale symptoms that may also appear with other engine problems.
- Rough idle or engine vibrations at low speed
- Reduced power and sluggish acceleration
- Increased fuel consumption and higher exhaust emissions
- Engine misfire sensations under load or during acceleration
- Check Engine Light or MIL (malfunction indicator lamp) illuminated
Because these signs overlap with other issues, proper diagnostic testing is essential to confirm a cylinder 3 misfire.
Diagnosing P0303 on a Lexus
Effective diagnosis combines data from a scan tool with hands-on component checks to identify root causes.
- Use an OBD-II scanner to confirm P0303, review freeze frame data, and check for related codes like P0300 (random misfire) or P0301–P0308 (cylinder-specific misfires).
- Inspect the cylinder 3 ignition system: remove and inspect the spark plug, test or swap the ignition coil/coil-on-plug for cylinder 3 to see if the misfire follows the part.
- Check the fuel system for cylinder 3: test injector resistance, listen for injector operation, and verify fuel pressure is within specification.
- Look for air leaks around the intake tract and cylinder 3 region; inspect intake gaskets and vacuum lines.
- Perform a compression test on cylinder 3 and compare to neighboring cylinders to assess mechanical health.
Documenting findings helps determine whether the issue is electrical, fuel-related, or mechanical, guiding appropriate repairs.
Common Fixes
Fixes depend on the root cause identified during diagnosis. Typical repairs include:
- Replacing a worn spark plug in cylinder 3 and ensuring correct gap and heat range
- Replacing or swapping the ignition coil or coil-on-plug for cylinder 3 if it is faulty
- Cleaning, repairing, or replacing a malfunctioning fuel injector for cylinder 3; addressing fuel pressure if needed
- Repairing vacuum or intake leaks near cylinder 3, replacing gaskets or O-rings
- Addressing mechanical issues such as worn valves, timing problems, or poor compression with appropriate engine work
Because persistent misfires can damage catalytic converters and emissions systems, it's advisable to address P0303 promptly and follow through with a thorough repair.
When to Seek Professional Help
If the misfire persists after performing basic checks and common fixes, or if you experience stalls, loss of power, or unusual noises, consult a qualified mechanic or Lexus dealership. Advanced diagnostics, proper timing checks, and specialized equipment may be required for a definitive fix.
Summary
P0303 on a Lexus indicates a misfire in cylinder 3. It signals that the cylinder is not firing consistently, which can lead to rough running, reduced power, and higher emissions. Causes range from ignition and fuel delivery issues to mechanical problems and sensor or wiring faults. A structured diagnostic approach—scanning, component testing, and mechanical checks—guides effective repairs, which may involve spark plugs, coils, injectors, or more extensive engine work. Timely attention helps prevent further damage and protects emissions compliance.
