How to Fix Error Code P0303: Cylinder 3 Misfire Detected

Error code P0303 means your vehicle’s computer has detected a misfire in cylinder 3, usually caused by issues with the spark plug, ignition coil, fuel delivery, or compression in that cylinder; fixing it typically involves inspecting and replacing faulty ignition components, checking the injector, and ruling out mechanical problems.

What Error Code P0303 Really Means

P0303 is an OBD-II diagnostic trouble code that translates to “Cylinder 3 Misfire Detected.” Your engine’s control module (ECM/PCM) monitors crankshaft speed and other signals. When one cylinder fails to contribute normal power—because the air-fuel mixture is not igniting properly—the computer flags a misfire. If it determines the misfire comes specifically from cylinder 3, it stores P0303 and usually turns on the check engine light.

Why Misfires Matter

A misfire may seem minor at first, but it can quickly damage your catalytic converter, increase fuel consumption, and cause poor drivability. Prolonged misfires send unburned fuel into the exhaust, overheating and melting the catalyst. Addressing P0303 promptly helps avoid costly repairs and improves safety and reliability.

Common Symptoms of Code P0303

When P0303 is active, the vehicle often gives clear warning signs, from dash lights to performance changes. Recognizing these symptoms can help you confirm that the issue is more than just a momentary glitch.

• Illuminated check engine light (CEL), sometimes flashing under load
• Rough idle or noticeable engine shaking when stopped
• Hesitation, stumbling, or jerking during acceleration
• Loss of power, especially climbing hills or passing
• Increased fuel consumption and poor mileage
• Raw fuel smell from the exhaust, sometimes with popping
• In severe cases, stalling or hard starting

If you experience several of these symptoms alongside code P0303, it’s a strong indicator that cylinder 3 is not firing correctly and the issue requires diagnosis rather than being ignored.

Main Causes of P0303

P0303 can be triggered by a range of issues affecting ignition, fuel delivery, air intake, or mechanical integrity of cylinder 3. Understanding the most common causes helps you prioritize what to check first and avoid unnecessary parts replacement.

• Faulty spark plug in cylinder 3 – Worn, fouled, cracked, or incorrectly gapped plug.
• Defective ignition coil or coil-on-plug unit – Weak spark, internal short, or open circuit.
• Damaged plug wire or boot (if equipped) – Arcing, carbon tracking, or corrosion.
• Clogged or failing fuel injector #3 – Not delivering proper fuel quantity or pattern.
• Low fuel pressure – Failing pump, clogged fuel filter, or bad pressure regulator affecting all cylinders but triggering misfire on the weakest.
• Vacuum leak near cylinder 3 – Leaking intake manifold gasket, cracked hose, or PCV-related leak.
• Engine mechanical problems – Low compression due to worn piston rings, burned valve, head gasket leak, or damaged cam/lifter.
• Sensor or control issues – Faulty crankshaft or camshaft position sensor, or rare PCM/software problems mismanaging ignition or injection.
• Fuel quality issues – Contaminated or extremely low-quality fuel leading to poor combustion.

While ignition-related faults account for many P0303 cases, overlooking fuel and mechanical checks can lead to repeated failures, so a thorough approach is important.

Step-by-Step Diagnosis for P0303

Efficiently diagnosing P0303 involves going from the simplest, least expensive checks to more advanced tests, confirming the cause instead of guessing. The sequence below is similar to what many professional technicians follow.

1. Confirm the Code and Look for Related Issues

Before replacing any parts, verify exactly what the vehicle’s computer is reporting and whether other codes might point toward a root cause.

1. Scan the vehicle with an OBD-II scanner. Confirm P0303 is present and note any additional codes (e.g., P0300, P0301–P0306, fuel trim codes, oxygen sensor codes, or misfire counters).
2. Check freeze frame data. Look at RPM, load, coolant temp, and speed when the misfire occurred; this can show if it happens at idle, under load, or when cold.
3. Clear codes and test drive. After clearing, see if P0303 returns under specific conditions (e.g., accelerating, at idle, or highway speeds).

This initial scan establishes whether you’re dealing solely with a single-cylinder misfire or a broader issue involving fuel, timing, or emissions systems.

2. Inspect Ignition Components on Cylinder 3

The spark plug and ignition coil (or plug wire) on cylinder 3 are the most common culprits and are usually the easiest to inspect and test.

1. Locate cylinder 3. Consult your vehicle’s manual or a reliable repair database; numbering varies between inline, V6, and V8 engines.
2. Remove and inspect the spark plug. Look for oil fouling, carbon buildup, broken insulator, or a worn electrode; check the gap against manufacturer specs.
3. Swap components to a different cylinder. Move the spark plug and, if applicable, the coil or plug wire from cylinder 3 to another cylinder (e.g., cylinder 1).
4. Clear codes and test drive again. If the misfire code changes from P0303 to the cylinder you swapped with (e.g., P0301), the part you moved is likely faulty.
5. Check the coil connector and wiring. Inspect for loose pins, corrosion, or damaged insulation around the coil harness.

If the misfire “follows” the moved plug or coil to another cylinder, replacing that component usually resolves P0303; if not, the cause likely lies elsewhere.

3. Evaluate Fuel Delivery to Cylinder 3

If ignition checks out, the next focus is whether cylinder 3 is getting the right amount of fuel and whether it’s being delivered correctly.

1. Listen to injector #3. With the engine running, use a mechanic’s stethoscope or a long screwdriver to listen for a steady clicking noise from the injector.
2. Perform an injector balance or swap test (if possible). On some vehicles you can command injectors individually with a scan tool; otherwise, swap injector #3 with another cylinder and see if the misfire code follows.
3. Check fuel pressure. Use a fuel pressure gauge on the rail and compare readings to factory specs at idle and under load.
4. Inspect wiring to injector #3. Look for damaged connectors, broken wires, or corrosion at the injector plug.

When a misfire moves with the injector during a swap test or when injector operation is clearly abnormal, cleaning or replacing the injector is typically necessary to restore smooth operation.

4. Check for Vacuum Leaks and Air-Related Problems

Excess air in or around cylinder 3, especially unmetered air from a vacuum leak, can lean out the mixture enough to cause a misfire under certain conditions.

1. Visually inspect hoses and intake components. Check PCV hoses, brake booster lines, and the intake manifold area adjacent to cylinder 3 for cracks or loose connections.
2. Use a smoke test or spray test. A professional smoke machine is ideal; alternatively, carefully spray carb/brake cleaner around suspected areas at idle and listen for RPM changes.
3. Monitor fuel trims with a scan tool. High positive fuel trims at idle that normalize at higher RPMs often point to a vacuum leak.

Finding and sealing leaks near cylinder 3 can eliminate the lean misfire and often improves overall fuel economy and idle quality as well.

5. Test Engine Mechanical Health (Compression and Leakdown)

If ignition, fuel, and air systems appear normal, internal engine problems in cylinder 3 may be responsible for P0303, especially on higher-mileage vehicles or engines with known weak points.

1. Perform a compression test. With all plugs removed and the throttle held open, compare cylinder 3’s compression to the others; significant deviation (often more than 10–15%) is a red flag.
2. Do a leakdown test if compression is low. This helps pinpoint whether air is escaping through the intake valves, exhaust valves, piston rings, or head gasket.
3. Inspect for coolant or oil issues. Milky oil, white exhaust smoke, or unexplained coolant loss near cylinder 3 may indicate a head gasket problem.

Mechanical faults typically demand more extensive repairs—such as valve work, head gasket replacement, or internal engine rebuilding—making accurate diagnosis critical before committing to major work.

Common Repairs That Fix P0303

Once the root cause has been identified, the repair scope can range from simple plug replacement to complex engine work. Below are the most frequent fixes that resolve a cylinder 3 misfire.

• Replace the spark plug in cylinder 3 – Using the correct heat range and gap specified by the manufacturer.
• Replace the ignition coil or coil pack – Especially when a swap test shows the misfire follows the coil.
• Replace plug wires or boots (if equipped) – To eliminate arcing, carbon tracking, or intermittent connections.
• Clean or replace injector #3 – Using professional cleaning or installing a new injector when clogged or electrically faulty.
• Repair fuel system issues – New fuel filter, pump, or pressure regulator if pressure is outside spec.
• Fix vacuum leaks – Replace cracked hoses, PCV components, or intake manifold gaskets.
• Address sensor or PCM issues – Replace failing cam/crank sensors and update or reflash the ECM/PCM if a known software issue exists for your vehicle.
• Repair internal engine damage – Valve job, head gasket replacement, or more extensive engine repair if compression/leakdown tests confirm mechanical failure.

Choosing the correct repair depends entirely on accurate troubleshooting; blindly replacing parts can be costly and may not resolve the misfire if the true cause is overlooked.

Can You Keep Driving With P0303?

Driving briefly with a mild misfire is sometimes unavoidable, but continuing to operate the vehicle for days or weeks with P0303 active risks damage and safety issues. A flashing check engine light, strong fuel smell, or severe power loss signal a condition that should be addressed immediately, ideally by towing the car instead of driving it.

Preventing Future P0303 Misfires

Preventive maintenance significantly reduces the chances of cylinder-specific misfires like P0303 by keeping ignition and fuel systems in good working order.

• Follow factory intervals for spark plug and ignition component replacement.
• Use high-quality fuel and avoid running the tank extremely low.
• Replace air and fuel filters as scheduled to maintain proper mixture and pressure.
• Address minor roughness or hesitation early before it becomes a chronic misfire.
• Keep up with oil changes to prevent deposits on valves and pistons.

By staying ahead on routine service and reacting quickly to early symptoms, you can often prevent error code P0303 from appearing—or recurring—over the life of the vehicle.

Summary

Error code P0303 indicates a misfire in cylinder 3, most commonly caused by a bad spark plug, faulty ignition coil, or fuel injector issue, but sometimes by vacuum leaks or internal engine problems. Diagnosis starts with scanning for codes, inspecting and swapping ignition parts, checking injector function and fuel pressure, looking for vacuum leaks, and—if needed—testing compression. Repairs range from simple plug or coil replacement to injector service or engine work, depending on what tests reveal. Addressing P0303 promptly protects your catalytic converter, restores performance, and helps keep operating costs under control.

What is the most common cause of P0303?

And so what would be some possible causes of a P. 0303. Code well it could be a bad spark plug it could be a bad spark plug wire could be a bad coil. Could be a bad fuel injector.

How do you fix a misfire on cylinder 3?

A cylinder 3 misfire can be fixed by diagnosing and replacing faulty components like the spark plug, ignition coil, or fuel injector. If those are fine, the issue could be a vacuum leak or a more serious mechanical problem like low compression, a blown head gasket, or a damaged valve. The most common and cheapest fix is to start by checking the ignition system components, such as swapping the spark plug and coil with a working cylinder to see if the misfire follows.
 
This video demonstrates how to identify a cylinder 3 misfire: 0:591A Auto: Repair Tips & Secrets Only Mechanics KnowYouTube · Jun 30, 2023
Quick fixes to try first

• Spark plug: Inspect and clean it, or replace it with a new one. If the plug is wet or oily, it points to a more serious issue. 
• Ignition coil: Check for damage or stains. It’s a common failure point, so replacing it can often fix the problem. A simple test is to swap the coil from cylinder 3 with a coil from a different cylinder. If the misfire code changes to that new cylinder, you know the coil is bad. 
• Fuel injector: If cleaning doesn’t work, the injector may need to be replaced. 

If the quick fixes don’t work, investigate further

• Vacuum leaks: Check for any leaks in vacuum hoses or gaskets that could be disrupting the air-fuel mixture. 
• Compression test: If ignition and fuel delivery are ruled out, a compression test is the next step to check for mechanical problems. 
• Low compression: This could be caused by a worn piston ring, a valve issue, or a broken valve spring. These are more complex and may require professional repair. 
• Head gasket: A blown head gasket can also cause a misfire and is a more significant repair. 

Important considerations

• If the check engine light is on, a P0303 code indicates a cylinder 3 misfire. 
• If you have a misfire in multiple cylinders, it could point to a more general issue like a faulty fuel pump or sensor. 
• It’s recommended to diagnose the problem accurately before starting repairs, and an automotive scanner can provide valuable diagnostic data. 
• If you are not comfortable performing these repairs yourself, take the vehicle to a qualified mechanic. Ignoring the misfire can lead to more serious and expensive engine damage over time. 

How much does it cost to fix code P0303?

The cost to fix a P0303 code can range from under \$100 to over \$1,000, depending on the underlying cause. Simple fixes like replacing a spark plug or coil pack can cost a few hundred dollars or less, while more complex issues such as a cracked intake manifold, a bad fuel injector, or internal engine damage will be significantly more expensive. A professional diagnostic is needed to accurately determine the cause and final cost. 
Potential repair costs

• Simple fixes:
  • Spark plug/wire replacement: Expect a few hundred dollars, or less if done at home. 
  • Ignition coil replacement: Can range from under \$100 to a few hundred dollars. 
• More complex fixes:
  • Fuel injector replacement: Can cost up to \$400, depending on the vehicle. 
  • Intake manifold replacement: One example cites a \$1,800 repair, which included replacing a cracked manifold and a damaged coil. 
  • Engine internal damage (e.g., piston rings): A dealership quoted one driver \$4,500 for a piston ring replacement. 
• Diagnostic costs: A professional diagnosis can cost around \$150-$165, often credited toward the repair if you have the work done at the same shop. 

How to determine the repair cost

1. Get a proper diagnosis: A mechanic will need to perform a diagnostic test to find the root cause of the misfire. The P0303 code only tells you which cylinder is misfiring, not why. 
2. Consider the simplest solutions first: It is often recommended to start with the cheapest and most common fixes, such as replacing the spark plug and coil for cylinder 3. 
3. Address a leaky valve cover: A common cause on some engines is oil leaking from the valve cover into the spark plug tube, which can be fixed by replacing the valve cover gasket. This can cost around \$70 in parts, plus labor. 
4. Rule out other causes: A misfire can also be caused by a bad fuel injector or a problem with the engine’s timing. 
5. Consider a professional tune-up: If you’re not comfortable with DIY repairs, a professional tune-up can be a good way to get an estimate for a range of potential issues and have the work done at the same time. 

Does P0303 always mean replace parts?

This code can occur for multiple reasons. The most common cause relates to the vehicle’s ignition system components, especially if an engine tuneup is long overdue. The culprit could be a faulty spark plug, a wire or a failed ignition coil pack.