AutoCodes – GMC – P0303 GMC

P0303 GMC Code - Cylinder 3 Misfire Detected

Quick Answer

A misfiring cylinder causes slight variations in the rotational speed of the crankshaft. Common causes include Faulty Spark Plug #3, Clogged or faulty Fuel Injector #3, Faulty Ignition Coil #3. 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: P0303 GMC indicates Cylinder 3 Misfire Detected.
Common causes: Common causes include Faulty Spark Plug #3, Clogged or faulty Fuel Injector #3, Faulty Ignition Coil #3.
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.

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Code Popularity 35,773

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Jul 21, 2022

Description
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes ▶
Cost to Diagnose
FAQs
Comments
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Code P0303 GMC Description

A misfiring cylinder causes slight variations in the rotational speed of the crankshaft. The Engine Control Module (ECM) can detect these variations by closely monitoring the crankshaft position sensor signal. The signals of the camshaft position sensors are used to determine which cylinder is misfiring. If the ECM detects a misfire rate sufficient to cause emission levels to exceed mandated standards, the Diagnostic Trouble Code (DTC) will be set. Under certain driving conditions, a misfire rate can be high enough to cause the catalytic converter to overheat, damaging the converter. The Malfunction Indicator Lamp (MIL) flashes when the converter is overheating, damaging conditions are present, and the DTC is set. The DTC will be set when the ECM detects a crankshaft rotation speed variation indicating a single-cylinder misfire rate sufficient to cause emissions levels to exceed mandated standards.

OBDII Code P0303 GMC - Cylinder 3 Misfire Detected - AutoCodes.com — P0303 GMC Code - Cylinder 3 Misfire Detected

What does this mean?

What are the Possible Causes of the DTC P0303 GMC?

Faulty Spark Plug #3
Clogged or faulty Fuel Injector #3
Faulty Ignition Coil #3
Fuel Injector #3 harness is open or shorted
Fuel Injector #3 circuit poor electrical connection
Ignition Coil #3 harness is open or shorted
Ignition Coil #3 circuit poor electrical connection
Insufficient cylinder 3 compression
Incorrect fuel pressure
Intake air leak

How to Fix the DTC P0303 GMC?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

The P0303 code means that cylinder 3 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 3. 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 P0303
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.

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What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0303 GMC code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P0303 GMC code?

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Need Help with the Code?

Get the P0303 GMC code diagnosed by a professional: Find a repair shop in your area

Information for Specific GMC Models

P0303 2014 GMC Sierra — Cylinder 3 Misfire Detected
P0303 2015 GMC Sierra — Cylinder 3 Misfire Detected
P0303 2016 GMC Sierra — Cylinder 3 Misfire Detected
P0303 2017 GMC Sierra — Cylinder 3 Misfire Detected
P0303 2018 GMC Sierra — Cylinder 3 Misfire Detected

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 P0303 GMC Code

What does the OBDII code P0303 mean for a GMC vehicle?

P0303 indicates a misfire has been detected in cylinder number 3 of your GMC vehicleâ€™s engine.

What are common symptoms of a P0303 code?

Symptoms include rough idling, poor acceleration, decreased fuel economy, engine hesitation, and potentially an illuminated Check Engine Light.

What causes cylinder 3 misfires in GMC vehicles?

Common causes include faulty spark plugs, a bad ignition coil, clogged fuel injectors, vacuum leaks, low compression, or issues with the cylinder 3 wiring.

Can driving with a P0303 code damage my engine?

Yes, prolonged driving with a misfire can lead to engine damage, such as overheating or catalytic converter failure.

How do I diagnose a P0303 code?

Start by checking the spark plug, ignition coil, and fuel injector for cylinder 3. Additionally, inspect for vacuum leaks, poor compression, or wiring issues.

Can a faulty spark plug cause the P0303 code?

Yes, a worn or damaged spark plug in cylinder 3 can lead to a misfire and trigger the P0303 code.

Will replacing the ignition coil fix the P0303 code?

If the ignition coil for cylinder 3 is faulty, replacing it can resolve the issue. However, ensure proper diagnosis to confirm the coil is the cause.

What tools do I need to troubleshoot a P0303 code?

Youâ€™ll need an OBDII scanner, multimeter, spark plug tester, compression tester, and potentially fuel system cleaning tools.

How much does it cost to repair a P0303 code on a GMC?

Repair costs vary depending on the cause, ranging from $50 for a spark plug replacement to over $500 for more extensive repairs like fixing low compression.

Can bad fuel cause a P0303 code?

Yes, contaminated or low-quality fuel can cause misfires, including in cylinder 3, and may trigger the P0303 code.