P0303 – Meaning, Causes, Symptoms, & Fixes
Code P0303 Definition
Cylinder 3 misfire detected.
Commonly associated with codes: P0300
What Does P0303 Mean?
P0303 indicates that cylinder number 3 is experiencing misfires. A misfire occurs when an insufficient amount of fuel is burning in a cylinder. The efficient burning of fuel is essential to engine operation as the combustion of fuel is what provides the energy to power the engine. A misfire from one or more cylinders can be caused by many reasons from a faulty ignition system, fuel system, or internal engine failure. When P0303 occurs, it should be fixed immediately as long term driving with engine misfires could cause consequential damage to your engine. Many times, P0303 occurs when there are worn-out spark plugs, spark plug wires, or a faulty ignition coil.
What Are the Symptoms of Code P0303?
- Check Engine Light is on
- Check Engine Light flashing
- Engine runs rough and shaking
- Lack of power from the engine
- Fuel smell from the exhaust
- Hesitations/Jerking when accelerating
What Is the Cause of Code P0303?
(* = Most Common)
- Faulty or worn spark plugs *
- Faulty spark plug wires or coils *
- Distributor failure *
- Faulty fuel injector
- Vacuum leak
- Low fuel pressure
- Camshaft sensor defective
- Crankshaft sensor defective
- Engine timing off
- Leaking head gasket
- Low engine compression
- Poor quality fuel
How Serious Is Code P0303? – Severe
P0303 should be repaired immediately. Ignoring this error could result in ignition failure, catalytic converter damage, and unsafe/dangerous conditions while operating the vehicle.
Code P0303 Common Diagnosis Mistakes
Loose-fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.
Tools Needed to Diagnose Code P0303:
- Digital multimeter
- 5/8in. Spark Plug Socket
- Ratchet, sockets, and extensions
- Fuel pressure gauge
- Compression tester
- Leakdown tester
- Spark plugs
- Spark plug wires
How To Diagnose Code P0303:
Difficulty of Diagnosis and Repair – 2 out of 5
- Use FIXD to scan your vehicle to verify P0303 is the only code present. If other codes are present, they must be addressed first.
- Check for loose connectors at the ignition coils or for damaged wiring (specifically cylinder 3). Look for loose engine ground wires as well. These can cause random misfire conditions. Tighten or connect where necessary.
- Check the condition of your spark plugs and spark plug wires. If your vehicle is equipped with individual coil packs instead of spark plug wires, start by removing the cylinder 3 coil and swapping it with the cylinder number 4 coil. If the misfire moved to cylinder number 4 (P0304), then you have determined that the coil pack is faulty and needs replacement. You can do the same test with the spark plug, i.e., move cylinder 3 plug to say cylinder 2 and if the misfire is now P0302 then the plugs are at fault. Replace spark plugs and spark plug wires/ignition coil packs if needed and recheck for misfires. (How to identify a fouled spark plug, How to test spark plug wires, How to gap spark plugs, How to replace spark plugs, How to swap ignition coils)
- If you have determined that your ignition system is operating correctly, there may be a problem within your fuel system that is causing the random misfires. The following should be checked to ensure the engine is getting the proper amount of fuel: Check fuel pressure. Low fuel pressure can cause intermittent misfires on multiple cylinders. When the pressure is below the specification, the engine does not receive the proper amount of fuel and will start to lean misfire. The fuel pump or fuel pressure regulator could be the source of the low fuel pressure. (How to check fuel pressure). Check that the fuel injectors are functioning properly and activating. Random misfires can be a sign of faulty or clogged fuel injectors that need to be replaced. Also, check that the fuel injector wiring is not damaged and is connected properly. (How to check fuel injectors by ear, How to check fuel injectors with a digital multimeter).
- If the ignition system and fuel system checks out, you may want to perform an engine compression test and leak down test to see if there are any mechanical problems causing your misfire. Some common mechanical problems that cause misfire can be:
- Leaking head gasket
- Broken valve spring
- Broken piston ring
- Worn valve guides
- Burned valve
- Timing chain or belt skipped tooth and engine is off time.
Estimated Cost of Repair
For error code P0303, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.
- Spark plugs $40-$640 (some cars require Intake manifold removal)
- Ignition Coils $230-$640 (some cars require Intake manifold removal)
- Spark plug wires $180-$240
- Fuel injectors $1500-$1900
- Vacuum leak $100-$200
- Fuel pump $1300-$1700
- Fuel pressure regulator $200-$400
Ultimate Car Repair Report
Whether you’re planning to DIY repairs or go to a nearby mechanic, this free report contains valuable information for all drivers. Download The Savvy Motorist Report now to discover:
✅ 6-step checklist to get the best price on repairs
✅ 8 mechanic scams and how to avoid them
✅ How to instantly diagnose a check engine light on your smartphone
✅ And much more