What Does P0303 Mean?
The ECM has detected a misfire condition specifically in cylinder 3. The crankshaft position sensor detects a deceleration in crankshaft rotational velocity during the cylinder 3 power stroke, indicating that cylinder is not producing its expected torque contribution. The driver may notice rough idle, vibration, hesitation on acceleration, and a blinking or steady check engine light.
Safety Warning
A misfiring cylinder sends unburned fuel to the catalytic converter, causing overheating and potential failure. If the check engine light is flashing, minimize driving and avoid high load. Steady light with mild symptoms is less urgent but should be addressed within a week.
Common Causes
30%
Worn or fouled spark plug on cylinder 3 unable to produce adequate spark
25%
Failing ignition coil on cylinder 3 producing weak or intermittent spark
20%
Fuel injector on cylinder 3 restricted or malfunctioning
15%
Compression loss on cylinder 3 from worn rings, leaking valve, or head gasket breach
10%
Vacuum leak at the cylinder 3 intake runner causing a lean misfire
Diagnostic Steps
1
Swap the ignition coil from cylinder 3 with an adjacent cylinder — if the misfire follows the coil, replace it; if it stays on cylinder 3, the coil is not the cause.
2
Remove and inspect the cylinder 3 spark plug — check gap (typically 0.028-0.035"), electrode condition, and fouling pattern to narrow down the root cause.
3
Check cylinder 3 fuel injector with a stethoscope or Noid light — verify it clicks rhythmically at idle; measure resistance (typically 11-18 ohms) and compare to adjacent cylinders.
4
Perform a compression test on cylinder 3 — minimum is typically 100 PSI and should be within 10% of the highest cylinder; low compression warrants a leak-down test to find the leak source.
5
Monitor cylinder 3 misfire counter in live data while performing swap tests to definitively identify the failing component.
Estimated Repair Cost
$50 - $800
Parts + labor, varies by vehicle and location
The ECM has detected a misfire condition specifically in cylinder 3. The crankshaft position sensor detects a deceleration in crankshaft rotational velocity during the cylinder 3 power stroke, indicating that cylinder is not producing its expected torque contribution. The driver may notice rough idl...
The most common cause of P0303 (Cylinder 3 Misfire Detected) is: Worn or fouled spark plug on cylinder 3 unable to produce adequate spark
Typical repair costs for P0303 range from $50 to $800, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A misfiring cylinder sends unburned fuel to the catalytic converter, causing overheating and potential failure. If the check engine light is flashing, minimize driving and avoid high load. Steady light with mild symptoms is less urgent but should be addressed within a week.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0303 to identify the root cause.
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Quick Info
Category
Powertrain
System
Ignition System
Difficulty
Type
Generic (SAE)
Recommended Tools
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