What Does GM Code P0303 Mean?
- P0303 definition: Cylinder 3 misfire detected.
- Issue Severity: SEVERE – Stop driving immediately.
- Repair Urgency: Fix this code immediately (same-day if possible) to avoid ignition failure, catalytic converter damage, and dangerous conditions.
- Diagnosis: A cylinder 3 misfire can be caused by anything from faulty spark plugs to low engine compression. Because there are so many variables that could cause a misfire, the best cost savings is to find a trustworthy repair shop to diagnose code P0303 as quickly and accurately as possible.
Find a Certified Shop
We've partnered with RepairPal to recommend trustworthy shops in your area. Enter your details to see certified shops near you that offer upfront estimates, guaranteed fair pricing, and a minimum 12-month, 12,000-mile warranty.
Your GM moves when gasoline is burned and power is generated inside a chamber known as the cylinder. Most engines have a 4, 6, or 8-cylinder engine, where more cylinders typically mean more power. Power is generated by pistons that move up and down while fuel is ignited at very specific times. A misfire typically occurs when the timing of this ignition is off. P0303 indicates that cylinder number 3 is experiencing misfires.
GM Code P0303 Causes
Cylinder 3 misfires can be caused by many reasons from a faulty ignition system, fuel system, or internal engine failure. The most common reason for this to happen is faulty or worn-out spark plug coil packs, especially if it’s been a while since you had a tune-up.
- 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
GM P0303 Symptoms
- 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
- Commonly associated with code P0300
How Do I Fix GM Code P0303?
With a misfire fault, the first step is to get it diagnosed to figure out what is causing the engine to misfire. If your vehicle is misfiring and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal certified shop nearby to pinpoint the problem and give an accurate estimate for repairs.
These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.
How Much Does It Cost to Fix GM Code P0300?
P0303 can be caused by anything from old spark plugs to vacuum leaks to poor engine compression. It’s impossible to give an accurate estimate without properly diagnosing the issue first.
If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your GM P0303 code.
Other Possible Repairs for GM Code P0303
Once properly diagnosed, P0303 may require one or more of the following repairs to resolve the underlying issue. These prices are based on national averages and include parts and labor. Your cost may differ depending on your location and type of vehicle.
- Spark plugs: $66-$250
- 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
Steps to Diagnose GM Code P0303:
Engine code P0303 could be caused by a number of things, including faulty spark plugs, faulty ignition system, distributor failure, and more. If you’d like to try to fix this fault code at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Keep in mind this is an intermediate-level diagnosis and repair and not recommended for beginners. Diagnosis requires more specialized equipment beyond what the FIXD Sensor can provide and it can be a time and labor-intensive process for inexperienced DIYers.
DIY difficulty level: Intermediate
This repair requires mechanical knowledge and is not recommended for beginners.
Tools/parts needed (our top picks from Amazon):
- Digital multimeter
- 5/8in. Spark Plug Socket
- Ratchet, sockets, and extensions
- Fuel pressure gauge
- Compression tester
- Leakdown tester
- Spark plugs
- Spark plug wires
Step 1: Use FIXD to scan your GM to verify P0303 is the only code present. If other codes are present, they must be addressed first.
Step 2: 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.
Step 3: 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)
Step 4: Check for fuel system issues.
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).
Step 5: Perform an engine compression and leakdown test.
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.
GM Code P0303 Common Diagnosis Mistakes
Loose-fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked.
Still Need Help Fixing GM Code P0303?
If you’ve followed the steps above and are still experiencing misfires or check engine code P0303, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.
At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.