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2006 Hummer H3 P0303 Misfire: Causes, Diagnostics & Solutions for Stalling at Stop

Model: Hummer H3 Fault Code: P0303 Posted: 2019-03-11 12:32

I own a 2006 Hummer H3 that stalls when coming to a complete stop. I've been getting a persistent P0303 diagnostic code indicating a misfire on cylinder 3. After replacing all spark plugs and coil packs, cleaning the throttle body, and checking for vacuum leaks, the issue remains unresolved. I'm trying to determine whether the stalling is directly caused by a faulty injector in cylinder 3 or if another component—such as the EVAP system—is at fault. The truck runs fine when idling or moving, but once warmed up and stopped (especially in traffic), it begins to stall intermittently. I recently added BG 44K fuel treatment, which improved performance temporarily—but now the stalling has become intermittent instead of constant. I suspect that old gasoline residue at the bottom of the tank may have clogged cylinder 3's injector, especially after running the engine down to near empty and refilling with stale fuel. I've also noticed a P0455 EVAP leak code, which I believe could be related—particularly since some reports indicate that a faulty canister valve solenoid in vehicles with similar engines (like the Chevrolet Colorado) causes intermittent stalling and misfires. My local auto parts store employee mentioned this as a common issue. I'm seeking confirmation from other 2006 Hummer H3 owners: is a single cylinder misfire typically enough to cause engine stalling? Could an EVAP system failure—especially the canister valve solenoid—trigger both a P0455 code and intermittent stalling at idle? I'm considering replacing the canister valve solenoid next, as it seems like a plausible fix. Any real-world experience with this issue on the 2006 Hummer H3 would be greatly appreciated.

Related fault codes
P0303
Comments (9)
Anonymous 2019-03-12 10:03

A cylinder misfire can stem from three main causes: 1) Ignition: Check spark plugs (especially for AC Delco models). Is cylinder #3 firing? Examine plug condition—wet, dry, black, or gray? 2) Fuel delivery: Inspect the injector for leaks, pulsing issues, or partial blockage. 3) Mechanical factors: Compression and vacuum leaks—less likely but worth checking. Try adding a can of Sea Foam or Techron fuel cleaner to the tank. Ensure your engine isn't using a K&N air filter (which may affect airflow). When stopping, shift into neutral—does the stalling persist? This helps isolate whether it's transmission-related.

Anonymous 2019-03-12 10:35

I replaced the spark plugs with new ones and used Sea Foam. It helped for a while—but the P0303 code returned. I suspect that when the engine runs down to low fuel levels, old gasoline residue clogs cylinder 3's injector. The truck ran fine until I drained it nearly to empty, then sat with stale fuel before refilling. After adding Sea Foam, performance improved temporarily—only to return once the tank was re-filled. This suggests a possible injector blockage due to sediment buildup in the fuel tank.

Anonymous 2019-03-12 12:58

Accurate diagnosis requires live data from an OBD2 scanner—including ECT, MAF, MAP, TPS, fuel trims, and oxygen sensor voltages—along with freeze frame data. A single cylinder misfire should not cause engine stalling. If the truck stalls at idle, it's unlikely to be a simple injector or ignition issue. What brand of spark plugs did you install? Did you confirm whether a K&N air filter is in use? Try shifting into neutral when stopping—this can help determine if transmission-related issues (like TCC engagement) are involved. In some cases, DIYers swap injectors and coil packs between cylinders to see if the misfire code follows the component—a useful diagnostic step.

Anonymous 2019-03-12 13:10

My engine is stock—no modifications. I've cleaned the throttle body, replaced the alternator, and swapped out all coil packs and spark plugs (Bosch, not AC Delco). The issue persists only on cylinder 3. I also get a P0455 EVAP leak code. I replaced the purge solenoid, but I haven't changed the canister valve solenoid yet. It seems odd that one injector could cause stalling—especially since the truck runs smoothly otherwise. Could the EVAP system be shutting off fuel flow at idle? That would explain why it only stalls when stopped and after warming up.

Anonymous 2019-03-12 15:22

A single cylinder misfire should not result in engine stalling. If the truck stalls during stops, shifting into neutral may reveal a transmission control issue (such as TCC engagement). Check your scanner—some basic scanners only show generic codes. If you have limited access to full diagnostic data, consider using a more advanced OBD2 scanner. Injector failure is one of three possible causes of misfire. Swapping the injector to another cylinder can help confirm whether it's faulty—the code would move if the injector is defective. In professional repair shops, injectors are typically replaced due to high labor costs (around $100/hour). Also check compression in cylinder #3. Note: Bosch spark plugs and K&N filters are not recommended for this engine model. What specific EVAP-related codes do you have? The purge valve may be a clue.

Anonymous 2019-03-12 17:34

I receive a P0455 code. My scanner only displays basic diagnostic trouble codes and does not show live data or freeze frame information.

Anonymous 2019-03-12 18:25

The stalling is intermittent—sometimes I shift into park to prevent it from happening, which helps. I've tested all gears with no issues when driving. The gas cap was visible inside the truck when it stalled, suggesting a possible EVAP leak. I only get P0303 and P0455 codes. Given that this engine shares components with the Chevrolet Colorado—a known model with canister valve solenoid failures—I believe the issue may lie there. I’ve heard reports of the valve solenoid causing full fuel pump shutdown, which could explain both stalling and misfires. This seems like a likely culprit for my symptoms.

Anonymous 2019-03-13 07:21

I recommend: 1) Visit an auto parts store for free OBD2 scans—your current scanner may not capture full diagnostic data. Copy all available codes. 2) If P0455 or another EVAP leak appears, perform a system leak check using a smoke machine (many shops offer this service). Also check TSBs (Technical Service Bulletins) for known issues. 3) Conduct a compression test on cylinder #3 and share the results—this will help rule out mechanical failure.

Anonymous 2019-03-14 14:00

I now have intermittent stalling. After adding BG 44K, performance improved—but the issue became less constant. I did research and found that vehicles with similar engines (like Chevrolet Colorados) commonly suffer from canister valve solenoid failure. My auto parts store staff confirmed this—when the valve is clogged or the solenoid fails, it can cause intermittent stalling and misfires. Based on this, I believe I’ve identified the root cause: replacing the canister valve solenoid should resolve both the P0455 code and the stalling issue.