What Does GM Code P0300 Mean?
- P0300 definition: Random, Multiple 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 multiple 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 take your car into a shop to have it diagnosed as quickly and accurately as possible.
Your vehicle 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. GM P0300 indicates that two or more cylinders are experiencing misfires.
GM P0300 Causes
Multiple 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 and/or spark plug wires
- Ignition issues, including failing or damaged ignition coils
- Distributor failure
- Faulty fuel injector
- Vacuum leak
- Low fuel pressure
- Camshaft and/or crankshaft sensor defective
- Engine timing off
- Leaking head gasket
- Low engine compression
- Poor quality fuel that is old or contaminated
GM P0300 Symptoms
- Check Engine Light is on or flashing
- Engine runs rough, hesitates, or jerks when accelerating
- Drivers may not notice any adverse conditions when driving
- In some cases, drivers may experience decreased fuel economy, fuel smell from exhaust, rough idling, or lack of power from the engine
- Commonly associated with error codes: P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308
How Do I Fix GM Code P0300?
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?
P0300 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 P0300 code.
Possible Repair Costs for GM P0300
Once properly diagnosed, P0300 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
DIY Steps to Diagnose GM Code P0300
Engine code P0300 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 code P0300 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 ENSURE NO OTHER ENGINE CODES ARE PRESENT.
Use FIXD to scan your GM vehicle to verify P0300 is the only code present. If other codes are present, they must be addressed first.
STEP 2: CHECK FOR LOOSE CONNECTORS OR DAMAGED WIRING AT THE IGNITION COILS.
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.
Worn and old spark plug wires are common causes of random misfires. Replace spark plugs and wires if needed and recheck for misfires.
STEP 4: CHECK TO ENSURE YOUR ENGINE IS GETTING THE PROPER AMOUNT OF FUEL.
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.
- Check fuel injectors: Make sure 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.
STEP 5: PERFORM ENGINE COMPRESSION AND LEAKDOWN TEST
Common mechanical problems that can cause misfires:
- 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.
Common GM P0300 diagnosis mistakes
Loose fitting electrical connectors and broken or disconnected vacuum hoses are often overlooked. Oxygen sensor(s) are another common misdiagnosis for P0300.
Still Need Help Fixing GM Code P0300?
If you’ve followed the steps above and are still experiencing misfires or check engine code P0300, 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.
Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals