The P0303 is one of several misfire codes you can read on your scan tool. I studied 120+ different vehicles with misfires and how were they fixed, to provide you with a list of the most common causes, also with an action plan to diagnose this fault in no time.
You don’t want to delay fixing your P0303 as the misfire will damage your catalytic converters or engine internals. Both of these fixes are priced at thousands of dollars while the misfire is often caused by a worn-out $5 spark plug. Fix it ASAP.
P0303 quick overview
| Meaning | P0303: Cylinder #3 Misfire Detected |
| Is it serious? | Yes, this code is serious and should be addressed immediately to prevent costly damage to the engine or catalytic converter. |
| Possible causes | -Worn or faulty spark plug in cylinder #3 -Defective ignition coil for cylinder #3 -Malfunctioning fuel injector for cylinder #3 -Timing issues (e.g., timing belt misalignment) -Vacuum leak affecting cylinder #3 -Internal engine issues (e.g., valve problems or damaged head gasket) -Wiring or electrical connection problems related to cylinder #3 |
| How to diagnose? | Inspect Spark Plug: Check the spark plug in cylinder #3 for signs of wear or fouling. Test Ignition Coil: Swap the ignition coil with another cylinder to see if the misfire moves. Check Fuel Injector: Perform a swap or test of the fuel injector in cylinder #3 to confirm it’s functioning correctly. Inspect Vacuum System: Check for vacuum leaks, particularly around the intake manifold and PCV valve. Perform Compression Test: Conduct a compression test on cylinder #3 to rule out internal mechanical issues. Inspect Timing Components: Examine the timing belt or chain and related components for misalignment or damage. Inspect Wiring: Examine wiring and connections related to cylinder #3 for any faults or loose connections. |
P0303 Meaning
The P0303 code shows that your car’s engine isn’t burning fuel properly in cylinder #3. This can cause the engine to run poorly and might lead to more expensive damage if not fixed soon. Start by checking the spark plug, ignition coil, and fuel injector in that cylinder.
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Case study: real cases of solving P0303
I checked specifically for code P0303 and picked these example fixes from my full case study of 123 vehicles with misfires.
How to fix P0303 on Dodge
Car: 1998 Dodge Durango 5.2L
Problem: Misfire, P0301, P303, P201, P203
Diagnosing: Freeze frame data displayed that fault happened while idling. Both fuel and ignition systems looked without issues, also there wasn’t any sign of vacuum leaks. The running compression test uncovered the problem.
Fix: Final fix was done by replacing the engine head valves.
How to fix P0303 on Lexus
Car: 1997 Lexus ES300 3.0L
Problem: Misfire, P0303
Diagnosing: The customer complained that the check engine light was illuminated after a heavy load on the vehicle, which indicates the fuel system problem in most cases. The technician previously changed the coils/plug, now he tried to scope injectors which didn’t show any issue.
Fix: Turned out the injector for cylinder #3 was plugged, and replacing it fixed the issue.
How to fix P0303 on Subaru
Car: 2002 Subaru Impreza WRX 2.0L
Problem: Hesitation, Surge, Cuts out, Misfire, P0301 – P0304
Diagnosing: The technician changed the timing belt, ignition coils, and spark plugs which seemed to fix the problem, but the car was back after a few hundred miles. The issue was hard to diagnose since it was intermittent, but the technician found the problem with the scope by scoping every pin of PCM.
Fix: New CKP (crank position sensor) fixed the issue.
How to fix P0303 on Hyundai
Car: 2007 Hyundai Entourage SE 3.8L
Problem: Misfire, P0303
Diagnosing: The problem was intermittent and cleaning the fuel system or changing the coils/plugs did not help. The fix was done by following the service bulletin and is a little weird because the oxygen sensor failure could possibly affect multiple cylinders but this was the fix.
Fix: Change the upstream oxygen sensor
How to fix P0303 on Toyota
Car: 2000 Toyota 4Runner Limited 3.4L
Problem: P0300, P0303
Diagnosing: Car had previously timing job done and the issue was hiding there. Freeze frame data showed the issue is happening at idle.
Fix: Re-torque crankshaft pulley.
P0303 FAQ and conclusion:
Check the Wikipedia article on combustion engines if you are still unsure what exactly the misfire is. Understanding it will help you with your diagnosis.
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I test OBD-II scanners and make DIY Engine diagnostics guides to help you solve your car problems without having to depend on the mechanic. A lot of them will try to scam you or are just no help at all. About Juraj Lukacko