P0303
Cylinder 3 Misfire Detected
The appearance of the standard OBD2 trouble fault code P0303 is an indicator that your vehicle ECU triggered a threshold alert for "Cylinder 3 Misfire Detected". Understanding the root component breakdown helps avoid expensive diagnostic fees.
Driver's Summary
Your vehicle's computer logged P0303 after detecting a malfunction in the cylinder 3 misfire detected system. On the road, this usually shows up as power loss, jerky acceleration, ticking noise. This is not a code to ignore — the underlying fault can rapidly worsen and lead to costly repairs if driving continues.
Symptoms
Power loss, jerky acceleration, ticking noise
Common Causes
- Fouled spark plug
- Ignition coil failure
- Vacuum leak near cyl 3
- Engine mechanical fault
How to Fix
- 1 Replace spark plug
- 2 Test coil pack
- 3 Inspect vacuum lines
- 4 Perform leak-down test
Technical Explanation
The PCM triggers P0303 after its internal monitoring routine detects that a specific circuit or sensor has exceeded its acceptable operating range. The PCM monitors crankshaft rotational velocity via the CKP sensor at a resolution of individual tooth gaps on the reluctor ring. A combustion event in each cylinder produces a measurable acceleration spike; its absence or weakness is flagged as a misfire event within a 200-revolution or 1000-revolution test window. Once confirmed, the code is stored as a permanent DTC and the MIL is activated. The freeze frame snapshot — recording RPM, load, coolant temperature, and fuel trim at fault detection — is also saved and is critical for accurate diagnosis.
Is It Safe to Drive?
With P0303 active, your engine or transmission is not operating within design parameters. Short-term driving may seem fine, but internal damage is accumulating — particularly to fouled spark plug.
Mechanic's Pro Tip
The fastest isolation technique for P0303 is the coil swap test: move the ignition coil from the affected cylinder to a neighboring cylinder and clear the code. If the misfire follows the coil, it's the coil. If it stays on the same cylinder, focus on the spark plug, injector, or compression. Never replace coils without also replacing the spark plug in that cylinder — a fouled plug will kill a new coil within weeks.
Coil replacement: $150 - $300