P0303 – Cylinder 3 Misfire Detected

DTC P0303 is an ISO/SAE controlled, General powertrain code that indicates the engine control module has identified a misfire event on Cylinder 3. A misfire means Cylinder 3 is not producing normal combustion torque during its firing event, which can lead to rough running, reduced power, and increased emissions. The code does not name a failed part; it only identifies the affected cylinder. P0303 is typically set when the controller detects abnormal crankshaft speed variation during the Cylinder 3 contribution window, using crankshaft position sensor input to evaluate combustion consistency. Because ignition faults, fuel delivery problems, air leaks affecting one cylinder, or internal mechanical issues can all produce the same misfire signature, accurate diagnosis requires testing instead of parts swapping.

P0303 Quick Answer

P0303 – Cylinder 3 Misfire Detected means the control module has detected that Cylinder 3 is not contributing normal combustion. Verify which cylinder is Cylinder 3 on your engine, review freeze-frame data, then test ignition, fuel delivery, air/vacuum integrity, and cylinder mechanical condition to determine why Cylinder 3 is misfiring.

What Does P0303 Mean?

P0303 – Cylinder 3 Misfire Detected is a General, ISO/SAE controlled powertrain diagnostic trouble code indicating the control module has detected misfire activity assigned specifically to Cylinder 3. In practical terms, the engine is not burning the air-fuel mixture consistently in Cylinder 3, so the cylinder’s torque contribution is lower or irregular compared to what the controller expects.

This DTC is cylinder-specific, not component-specific. P0303 does not confirm a bad spark plug, ignition coil, fuel injector, vacuum leak, or low compression by itself. It only confirms that the misfire detection logic has associated the misfire with Cylinder 3, and further checks are required to identify the root cause.

Theory of Operation

Misfire detection is typically based on crankshaft rotational behavior. When a cylinder fires normally, the crankshaft speeds up slightly during that cylinder’s power stroke. The engine controller monitors crankshaft position sensor data and evaluates the minute changes in crankshaft speed across each firing event. If the acceleration pattern that corresponds to Cylinder 3 repeatedly falls outside the expected range, the controller registers misfires for that cylinder and may store P0303.

A Cylinder 3 misfire can result from anything that disrupts combustion on that cylinder: insufficient spark energy at the correct time, incorrect air-fuel mixture, inadequate fuel delivery, excessive dilution of the mixture, or reduced compression/poor cylinder sealing. Because different fault types can produce similar crankshaft speed variation, a structured diagnostic approach is required to avoid replacing good parts.

Symptoms

• Check Engine Light illuminated (may flash when misfire is severe)
• Rough idle or noticeable engine shake
• Hesitation, stumble, or bucking during acceleration
• Reduced power, especially under load
• Unstable engine speed at idle
• Possible fuel odor from the exhaust if unburned fuel is present
• Additional diagnostic trouble codes may be stored (for example, other misfire-related or fuel/ignition-related codes)

Common Causes

• Ignition fault affecting Cylinder 3 (for example, spark plug wear/fouling, ignition coil output issue, or insulation/boot problems)
• Ignition circuit concern affecting Cylinder 3 (power, ground, or control circuit connection/wiring issues)
• Fuel injector concern for Cylinder 3 (electrical control issue, connection problem, restricted flow, or leaking injector)
• Air/vacuum leak influencing Cylinder 3 more than other cylinders (intake gasket leak, runner leak, or localized unmetered air)
• Fuel delivery issue that causes lean operation under certain conditions (fuel pressure/volume concern)
• Mechanical engine problem on Cylinder 3 (low compression, valve sealing issue, head gasket leakage, or ring/cylinder wear)
• Improper mixture dilution affecting Cylinder 3 (for example, abnormal distribution of crankcase vapors or exhaust gas where applicable)
• Engine control module output/driver concern for Cylinder 3 circuits (uncommon; verify wiring and loads before suspecting the module)

Diagnosis Steps

Tools you’ll want: a scan tool capable of viewing misfire counters/Mode $06 data and freeze-frame, a digital multimeter, appropriate wiring information, an ignition spark tester, a noid light or lab scope for injector command checks, a fuel pressure gauge (and the ability to observe pressure under load where possible), and a compression tester and/or leak-down tester. A smoke machine is helpful for intake/vacuum leak detection.

1. Confirm the code and capture data. Verify P0303 is present (pending, stored, or confirmed). Record freeze-frame data and note RPM, load, coolant temperature, fuel trims, and whether the event happened at idle, cruise, or acceleration.
2. Check for additional DTCs. Look for other misfire codes or codes related to ignition, injector control, fueling, air metering, or sensor power supply. Use those codes to guide test priority without changing the meaning of P0303.
3. Identify Cylinder 3 correctly. Confirm cylinder numbering and Cylinder 3 location for the specific engine configuration before performing any cylinder-specific testing or swapping.
4. Perform a focused visual inspection. Inspect Cylinder 3 ignition and injector connectors, harness routing, and terminal fit. Look for corrosion, loose pins, damaged insulation, oil/coolant contamination in plug wells, and any obvious intake leaks near the Cylinder 3 runner area.
5. Use scan data to verify the misfire is assigned to Cylinder 3. Review misfire counters (and Mode $06 where available) at idle and during a controlled drive under the conditions similar to freeze-frame. Confirm Cylinder 3 shows elevated counts relative to other cylinders.
6. Verify power and ground integrity for ignition/injection circuits. With circuits loaded (engine running or during appropriate command tests), perform voltage-drop checks on power and ground paths feeding the Cylinder 3 ignition and injector circuits. Address abnormal voltage drop, poor grounds, or supply issues before condemning components.
7. Test ignition performance on Cylinder 3. Verify spark is present and consistent. If appropriate and safe for the ignition system design, perform a swap test of Cylinder 3 ignition components with another cylinder and re-check misfire counters to see whether the misfire moves with the swapped component.
8. Test injector control and fuel delivery to Cylinder 3. Check injector electrical characteristics (compare to other injectors where applicable) and verify injector command with a noid light or scope. If your scan tool supports it, run a cylinder contribution/balance test. Confirm overall fuel pressure/volume meets specification, especially if the misfire occurs under load.
9. Check for localized air leaks and mixture issues. Smoke-test the intake system and inspect for vacuum/intake gasket leaks that could disproportionately affect Cylinder 3. Review fuel trims in the operating range where the misfire occurs to support or refute a mixture-related cause.
10. Verify mechanical integrity of Cylinder 3. If ignition and fuel checks do not identify the cause, perform a compression test and/or leak-down test on Cylinder 3 and compare results to other cylinders. Investigate any out-of-spec results before replacing electrical or fuel components.

Professional tip: Use freeze-frame plus misfire counters/Mode $06 to match your testing to the exact conditions that set P0303. A misfire that appears mainly at idle suggests different checks than one that appears mainly under load. Confirm circuit power/ground quality with voltage-drop testing under load before replacing ignition or injector parts, because poor connections can mimic component failure.

Possible Fixes

• Repair wiring, terminals, or connector fit issues affecting Cylinder 3 ignition or injector circuits
• Service or replace ignition components affecting Cylinder 3 after testing confirms ignition-related failure
• Service, clean (where appropriate), or replace the Cylinder 3 fuel injector after control and flow-related testing supports injector fault
• Repair intake/vacuum leaks or sealing issues that disproportionately affect Cylinder 3 air-fuel mixture
• Correct fuel pressure/volume problems that contribute to misfire under the relevant operating conditions
• Repair mechanical engine faults on Cylinder 3 (compression loss, valve sealing issues, head gasket leakage) when confirmed by mechanical testing

Can I Still Drive With P0303?

Driving with P0303 – Cylinder 3 Misfire Detected depends on misfire severity. If the engine is shaking heavily, power is significantly reduced, or the Check Engine Light is flashing, continued driving can increase the risk of catalytic converter damage and further engine issues, and the vehicle should be stopped and diagnosed as soon as practical. If the engine runs relatively smoothly and the light is steady, limit driving, avoid heavy acceleration and high load, and plan for prompt diagnosis and repair. The priority is to prevent sustained misfire operation and to correct the underlying cause.

How Serious Is This Code?

P0303 can range from moderate to severe depending on how often Cylinder 3 misfires and under what conditions. Intermittent, low-rate misfires may mainly affect idle quality and emissions, while frequent misfires can cause major drivability problems and may overheat the catalytic converter due to unburned fuel entering the exhaust. Persistent misfire can also contribute to poor performance and increased fuel consumption. Because severity is linked to misfire rate and operating conditions, treat P0303 as a timely repair item and verify the repair by confirming misfire counters remain controlled.

Common Misdiagnoses

Common misdiagnoses with P0303 include replacing ignition parts without confirming the fault follows the component, overlooking poor electrical connections or voltage-drop issues that interrupt coil or injector operation, and failing to confirm the correct Cylinder 3 location before swapping components. Another frequent error is skipping mechanical checks after ignition and fueling tests appear normal; low compression or valve sealing problems can produce a consistent cylinder-specific misfire that looks like an ignition or injector issue. Not using freeze-frame and misfire counter data can also lead to testing under the wrong conditions, causing missed intermittent faults.

Most Likely Fix

The most likely fix for P0303 – Cylinder 3 Misfire Detected is the repair that addresses the verified cause of the Cylinder 3 misfire after testing. In many cases, the resolution is found by confirming whether the misfire follows an ignition component swap, verifying injector command and delivery on Cylinder 3, correcting wiring/connection problems revealed by voltage-drop testing, repairing a localized air leak, or resolving a mechanical compression issue when test results show Cylinder 3 is not sealing properly. The correct “most likely” action is to follow the evidence from scan data and targeted tests, then confirm the misfire is eliminated.

Repair Costs

Repair cost depends on whether the confirmed root cause is wiring, connector condition, a sensor, a module, or the labor needed to diagnose the fault correctly.

Last updated: April 2, 2026