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Direct answer: P0303 is a confirmed cylinder 3 misfire detected by the engine control module. Fixing it requires methodical diagnosis: scan live data to confirm the misfire, check ignition (coil and spark plug), fuel delivery (injector and pressure), mechanical health (compression), and wiring/connectors — then replace or service the failing component.
Key Takeaways
- P0303 = Cylinder 3 Misfire Detected (GMC models including Sierra, Envoy, Acadia, Terrain).
- Common causes: bad coil pack, failed spark plug, clogged/failed injector, low compression, vacuum leak, or wiring problem.
- Diagnosis order: scan and capture freeze-frame/live data → swap tests (coil/injector) → compression and fuel pressure checks → wiring and PCM checks.
- Short drives with a light misfire may be possible, but a persistent or heavy misfire risks catalytic damage — get it scanned and fixed promptly.
What P0303 Means for a GMC Sierra
The generic DTC P0303 means the powertrain control module (PCM) logged a misfire specifically on cylinder 3. On GMC trucks and SUVs (Sierra, Envoy, Acadia, Terrain) the engine monitor looks at crank/cam sync and ignition/fuel events and reports when one cylinder is not producing the expected combustion. “Cylinder 3” is the location the PCM references in its firing order and misfire counters — it does not tell you the exact physical cause.
Typical Symptoms Drivers Notice
- Rough idle or shake at idle
- Loss of power, hesitation, or stumble under acceleration
- Check Engine Light illuminated, possibly flashing if misfire is severe
- Increased fuel consumption or black smoke from the exhaust
- Possible drivability codes for downstream systems (catalyst efficiency) if misfire continues
Common Causes (with what to look for)
- Ignition failure: faulty coil pack/ignition coil, burned or fouled spark plug, bad spark plug wire (if applicable).
- Fuel issue: clogged or failed fuel injector, injector wiring/connector fault, low fuel pressure from pump or regulator.
- Mechanical problems: low compression from worn rings, leaking valves, or head gasket issues on that cylinder.
- Air intake or vacuum leaks affecting that cylinder’s mixture.
- Sensor/wiring/PCM issues: cam/crank sensor irregularities or wiring harness damage; PCM faults are rare but possible.
Tools a Technician Will Use
- Scan tool with live data and misfire counters (not just a code reader)
- Multimeter for continuity and injector coil resistance checks
- Fuel pressure gauge
- Compression tester and/or leak-down tester
- Noid light for injector pulse testing
- Oscilloscope (optional, for advanced ignition and injector waveform analysis)
Step-by-Step Technician Diagnosis (recommended order)
- Read codes and freeze-frame data. Confirm P0303 and any related codes (P0300, fuel trims, O2/catalyst codes).
- Monitor live misfire counters while engine is idling and under load to confirm cylinder 3 is misfiring and to gauge severity.
- Visual inspection: unplug and reconnect cylinder 3 coil/ignition connector, inspect spark plug for fouling or damage, inspect wiring harness and injector connector for corrosion or broken pins.
- Swap test: swap the ignition coil from cylinder 3 with another cylinder’s coil (if coils are modular). Clear codes and test — if the misfire follows the coil, the coil is bad.
- If swapping the coil didn’t move the misfire, swap the fuel injector with another cylinder and retest — if code follows injector, replace injector or service it.
- Measure fuel rail pressure to ensure the rail and pump/regulator are within spec — a weak pump or regulator can cause lean misfires.
- Compression test or leak-down test on cylinder 3; compare to other cylinders. Low compression points to mechanical issues (rings, valves, head gasket).
- Check for vacuum leaks or intake gasket leaks near that cylinder, which can lean out the mixture locally.
- If tests are inconclusive, check cam/crank sensor signals and PCM data; consider oscilloscope traces for ignition and injector waveforms.
Quick Reference Table: Cause, Quick Test, Likely Fix
| Likely Cause | Quick Test | Typical Fix |
|---|---|---|
| Bad ignition coil | Swap coil with another cylinder; check for spark | Replace coil |
| Worn/fouled spark plug | Inspect and measure gap; check condition | Replace spark plug |
| Clogged/failed fuel injector | Noid light or swap injector | Replace or professionally clean injector |
| Low compression | Compression or leak-down test | Engine mechanical service required |
| Vacuum/intake leak | Spray carb cleaner or smoke test | Replace gasket/hoses; fix leak |
| Wiring/PCM | Continuity checks, inspect connectors, compare waveforms | Repair wiring or replace PCM if diagnosed |
Model-Specific Notes (GMC Sierra, Envoy, Acadia, Terrain)
Different GMC engines use different ignition designs (coil-on-plug vs coil packs) and injector topologies. The diagnostic order stays the same regardless of model: confirm the misfire with live data, then swap coils and injectors to isolate the faulty component. Searches like “dtc p0303 gmc” or “p0303 gmc envoy” often point to the same root causes listed above — the specific part (coil, plug, injector) will differ by engine.
When Is It Safe to Drive?
A single, light misfire that appears intermittently may allow a short drive to a safe location or shop. However, a persistent or heavy misfire (especially a flashing Check Engine Light) can quickly damage the catalytic converter, increase emissions, and create a fire risk if raw fuel enters the exhaust. Avoid extended highway driving or towing with an unresolved P0303.
How Much Will It Cost?
Cost depends entirely on the failed component and labor rates. Replacing a spark plug or coil is relatively inexpensive; injector replacement, head gasket, or internal engine work is much more. Get a full diagnostic scan first so you know which component failed before you get repair quotes.
Useful Next Steps
- Get a full scan with live data and freeze-frame; ask the shop to show misfire counters for cylinder 3.
- If you have a basic toolset, you can visually inspect coil and plug, but avoid guesswork on engine internals.
- If you’re shopping for a used GMC with this code history, consider a pre-purchase vehicle inspection to confirm current engine health and any misfire causes.
- For unrelated but important vehicle work — airbags, module resets, ADAS calibration or keys — see our vehicle safety system services page at https://lockncal.com/vehicle-safety-system-services/.
FAQ
Q: What does P0303 mean on a GMC Sierra?
A: P0303 means the PCM detected a misfire on cylinder 3 — the engine is not combusting correctly in that cylinder.
Q: Can I drive with a P0303 code?
A: Short distances to a shop may be safe for a light, intermittent misfire, but persistent or heavy misfires risk catalytic converter damage and unsafe driving conditions — get it checked promptly.
Q: Is P0303 caused by a bad spark plug or coil?
A: Often yes. Ignition components (coil, plug, wires) are common causes, but fuel injectors, compression issues, vacuum leaks, or wiring faults can also cause P0303.
Q: How does a mechanic find a cylinder 3 misfire?
A: They use a scan tool to confirm misfire counts, swap coils/injectors to see if the misfire follows the component, test fuel pressure and compression, and inspect wiring and sensors.
Q: If I clear the code, will it come back?
A: If the underlying cause isn’t fixed, the PCM will re-run its tests and the P0303 will usually return. Clearing the code only removes the warning temporarily.
If you want to schedule a diagnostic scan, module work, or any of our vehicle safety services, please schedule an appointment or call (469) 277-7277.
