P0303: Cylinder 3 Misfire Detected

What does P0303 mean?

The ECM watches crankshaft speed using the crankshaft position sensor. Each firing cylinder gives the crankshaft a small push. When cylinder three does not deliver that push often enough, the misfire monitor counts it and stores P0303.

Any engine with three or more cylinders can set P0303. On a 3 cylinder engine, cylinder 3 is the last one in the numbering.

Cylinder numbering depends on the engine, so confirm which cylinder is number 3 before testing parts. A single cylinder code narrows the search compared with P0300.

A light misfire may first appear as a pending code. A severe misfire can make the check engine light flash, which is a warning that unburned fuel may be overheating the catalytic converter.

If P0203 (Cylinder 3 Injector Circuit) is stored at the same time, check the electrical circuit of the cylinder 3 fuel injector first, because an injector that does not fire also makes that cylinder misfire.

Common symptoms

• Check engine light on or flashing
• Rough running or shaking, especially at idle
• Hesitation or jerking on acceleration
• Loss of power
• Higher fuel consumption

Common causes

The causes below are the ones commonly seen with P0303. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• A worn or fouled cylinder 3 spark plug, or a damaged plug wire or boot on that cylinder
• A failing cylinder 3 ignition coil (on coil on plug engines each cylinder has its own coil)
• A clogged, leaking or electrically faulty cylinder 3 fuel injector
• An intake or vacuum leak close to cylinder 3, such as a leaking intake gasket at that port
• Low fuel pressure or an EGR fault, which usually affect more than one cylinder
• Low compression on cylinder 3 from a valve, ring or head gasket problem (less common)

What to check with a scanner

• Read all codes. Fix lean codes (P0171, P0174) or fuel supply codes before chasing P0303.
• Use freeze frame data to see the rpm and load when the misfire happened.
• Where supported, check Mode 06 misfire counters to confirm which cylinder misfires and how often.
• Inspect the spark plug for wear, oil or coolant contamination.
• Swap the cylinder 3 ignition coil with cylinder 4, clear the code and drive. If the code changes to P0304, the coil is at fault. If P0303 comes back, look at the plug, the injector and compression on cylinder 3.

How to read and clear P0303 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0303 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0303 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0303.

When to see a mechanic

Get professional help if any of these apply:

• The check engine light is flashing
• The engine shakes badly or loses power
• You smell fuel or see smoke
• The misfire stays after replacing the plug and coil
• You suspect low compression or a timing problem

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand