A flashing check engine light on your 2022 Sonata is an emergency signal. Combined with P0303 indicating cylinder 3 is misfiring, your engine is experiencing combustion failures severe enough to damage the catalytic converter. This requires immediate attention.
Why the Light Flashes
A steady check engine light means the computer found a problem. A flashing light means active, severe misfires are happening right now. Each misfire sends unburned fuel to the catalytic converter, where it can ignite and cause overheating. Catalytic converter replacement can cost $1,500-$3,000.
What P0303 Means
The P0303 code pinpoints cylinder 3 as the problem cylinder. On the Sonata's inline-4 engines, cylinders are numbered 1-4 from the front. Cylinder 3 is second from the back.
Immediate Actions
- Reduce speed and load immediately
- Avoid hard acceleration
- Drive to the nearest safe location or shop
- If far from a shop, consider a tow
- Don't continue driving with a flashing light
Common Causes of Cylinder 3 Misfire
- Failed ignition coil: Coil pack for cylinder 3 has failed
- Worn spark plug: Plug in cylinder 3 is fouled or damaged
- Fuel injector failure: Injector clogged or stuck
- Vacuum leak: At cylinder 3 intake runner
- Compression loss: Mechanical issue with valves or piston
Quick Diagnosis: Coil Swap
- Swap cylinder 3 ignition coil with cylinder 4
- Clear the code
- Run the engine
- If code changes to P0304, replace the coil
- If P0303 remains, suspect plug or injector
Repair Costs
- Ignition coil: $75-$175
- Spark plug: $25-$50 (replace all 4)
- Fuel injector: $150-$350
- Vacuum leak repair: $100-$300
- Catalytic converter (if damaged): $1,500-$3,000
Your 2022 Sonata is under warranty—don't risk catalytic converter damage with continued driving.