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Engine-Codes – GMC – P0304 GMC

P0304 GMC Code - Cylinder 4 Misfire Detected

Quick Answer

When the diagnostic trouble code P0304 GMC appears, it indicates that a misfire has been detected in cylinder 4 of the engine. Common causes include Faulty spark plug, Defective ignition coil, Clogged or malfunctioning fuel injector. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P0304 GMC indicates Cylinder 4 Misfire Detected.
Common causes
Common causes include Faulty spark plug, Defective ignition coil, Clogged or malfunctioning fuel injector.
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Aug 25, 2024
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Code P0304 GMC Description

When the diagnostic trouble code P0304 GMC appears, it indicates that a misfire has been detected in cylinder 4 of the engine. This misfire can lead to a variety of performance issues, including rough idling, poor acceleration, decreased fuel efficiency, and potentially even engine stalling. A misfiring cylinder can cause damage to the catalytic converter over time if not addressed promptly.

OBDII Code P0304 GMC - Cylinder 4 Misfire Detected
P0304 GMC Code - Cylinder 4 Misfire Detected

Common Causes of P0304 GMC

  1. Faulty spark plug
  2. Defective ignition coil
  3. Clogged or malfunctioning fuel injector
  4. Vacuum leak
  5. Low compression in cylinder 4

Symptoms of P0304 GMC

  1. Rough idling
  2. Decreased acceleration
  3. Engine vibration
  4. Loss of power
  5. Engine stalling

How to Fix P0304 GMC

  1. Diagnose the specific cause of the misfire in cylinder 4 using a scan tool and performing a visual inspection of the spark plug, ignition coil, and fuel injector.
  2. Replace the faulty spark plug in cylinder 4 with a new one.
  3. Test the ignition coil in cylinder 4 and replace it if necessary.
  4. Clean or replace the fuel injector in cylinder 4 to ensure proper fuel delivery.
  5. Check for any vacuum leaks in the system and repair as needed.

Cost to Fix P0304 GMC

The cost of repairing a misfire in cylinder 4 can vary depending on the specific cause of the issue and the labor rates of the auto repair shop. On average, the repair cost for this issue can range from $100 to $500, including parts and labor. However, it's important to note that prices can differ based on location, vehicle make and model, and the extent of the repair needed. It's recommended to obtain a detailed quote from a reputable auto repair shop before proceeding with the repair.

What does this mean?

Tech Notes for P0304 GMC

The P0304 code means that cylinder 4 is misfiring or is randomly misfiring. Start by checking for intake leaks and if no leaks are found the next step is to replace the spark plugs on cylinder 4. If the problem persists more tests need to be done to diagnose the problem, see "Possible Causes".

There is a Factory Service Bulletins for All 2000-2020 GMC Models:
2000-2020 GMC Factory Service Bulletin OBDII Code P0304
This Factory Service Bulletin explains how this condition may be caused by significant carbon build-up on the intake and/or exhaust valves due to fuel contamination or incomplete burning of the fuel.

Need more information about the P0304 GMC code?

Need more information on how to fix the P0304 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P0304 GMC code diagnosed by a professional: Find a repair shop in your area

Information for Specific GMC Models

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0304 GMC Code

1. What are the common symptoms of OBDII code P0304 (Cylinder 4 Misfire Detected)?

Symptoms may include rough idling, engine vibration, loss of power, decreased fuel efficiency, and potentially, the illumination of the Check Engine Light.

2. What are some possible causes of OBDII code P0304 (Cylinder 4 Misfire Detected)?

Potential causes include faulty spark plugs, ignition coils, fuel injectors, vacuum leaks, low compression in the cylinder, or issues with the engine's computer system.

3. How can I diagnose the cause of OBDII code P0304 (Cylinder 4 Misfire Detected)?

Diagnosis typically involves inspecting and testing the spark plugs, ignition coils, fuel injectors, compression levels, and checking for vacuum leaks using a scan tool and performing a visual inspection.

4. Can driving with OBDII code P0304 (Cylinder 4 Misfire Detected) cause further damage to the vehicle?

Continuing to drive with a misfire can potentially lead to damage to the catalytic converter, oxygen sensors, or other engine components, so it is advisable to address the issue promptly.

5. What are some repair tips for fixing OBDII code P0304 (Cylinder 4 Misfire Detected)?

Repair tips may include replacing faulty spark plugs, ignition coils, or fuel injectors in cylinder 4, addressing any vacuum leaks, performing a compression test, and ensuring proper engine timing.

6. Is it safe to continue driving with the Check Engine Light on due to OBDII code P0304 (Cylinder 4 Misfire Detected)?

While the vehicle may still be operable, it is recommended to address the issue promptly to prevent potential damage and ensure optimal engine performance.

7. Can a DIY enthusiast fix OBDII code P0304 (Cylinder 4 Misfire Detected) at home?

Depending on the individual's mechanical skills and access to tools, some DIY enthusiasts may be able to address the misfire issue by following repair guides or tutorials specific to their vehicle.

8. How much does it typically cost to repair OBDII code P0304 (Cylinder 4 Misfire Detected) at a mechanic or dealership?

The cost of repairs can vary depending on the exact cause of the misfire, but it may range from a few hundred to over a thousand dollars, considering parts and labor expenses.

9. Are there any preventive measures to avoid OBDII code P0304 (Cylinder 4 Misfire Detected) in the future?

Regular maintenance, such as replacing spark plugs at recommended intervals, using quality fuel, and addressing engine performance issues promptly, can help prevent misfires and related trouble codes.

10. How long does it take to repair OBDII code P0304 (Cylinder 4 Misfire Detected)?

The repair time can vary based on the cause of the misfire and the complexity of the repair, but it may take anywhere from a few hours to a full day, depending on the extent of the issue and the mechanic's workload.

Discuss the causes and fixes for P0304 GMC OBDII Code

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  • Pls help ,I already change ,cap,rotor ,wires ,spark plugs and double check wires spark plugs conect,replacegas filter,add injectors cleaner and stil on PO304 ,gmc sonoma V6 '02 pls help !

    Reply to Comment

  • You could have a problem with a valve. I suggest performing a compression test.

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  • The problem i am having is this.... When i bought the truck it ran rough. Couple days later the truck decided not to run. I noticed smoke coming from the intake manifold. it melted and smoked real bad. I replaced the intake and gaskets and it ran fine for a couple weeks. Then the CEL starts flashing after 25 MPH. Now the CEL is always on and only flashes above 25 MPH. does not matter if it is hot or cold. I have replaced all plugs wires and coils. I replaced intake gaskets again and bench cleaned injectors. I am at a loss. Any ideas?

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  • What model and year is your vehicle? have you tried swapping the fuel injectors? besides the P0304 code any other codes?

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  • Title says 1999 GMC Sierra Lt. But when I buy parts some are required to be 2000 due to the manufacturing date. It has the 4.8 engine. Only other code is p0443 purge valve circuit.

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  • I would start by swapping the fuel injector. Sounds like the problem is either a bad fuel injector or the fuel injector connector. I checked a national database of of repair for you model and year, and other than the spark plugs, intake gasket, and wires, the fuel injector is the most common problem.

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  • So I finally had a chance to switch injectors 4 and 8. Code did not move with injector.

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  • Please Help! 1999 Gmc Jimmy 4.3l v6 misfire cylinder 4, replaced spider, cap and rotor, spark plugs and wires and MAF still failing misfire 4 what can I do??

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  • Other than the components you have replaced I don't any anything else setting a single cylinder misfire code, but I did find a bulletin with the following information. I hope that helps

    If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.

    The wire terminal attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.

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    Click here to enlarge image

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