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GMC Sierra 5.3L Engine Misfire P0306: Diagnosis and Solutions for Cylinder 6

Model: GMC Sierra 5.3L Fault Code: P0306 Posted: 2016-01-19 22:36

I'm experiencing a persistent P0306 fault code on my GMC Sierra with a 5.3L V8 engine. After replacing the spark plug, ignition wire, and coil pack for cylinder 6, the vehicle still exhibits symptoms including loss of power, engine shaking, and a flashing Check Engine Light. Occasionally, the truck runs smoothly, but during these episodes, the Stabilitrack light activates. The P0306 code indicates a misfire in cylinder #6. While I've replaced ignition components, the issue persists intermittently. Possible causes could include a faulty fuel injector, vacuum leaks, or a stuck lifter — especially given that this is a known issue with 5.3L engines. I'm seeking advice on next steps: Should I test for injector pulsing? Perform a compression test? Check for vacuum leaks? Additionally, how can I confirm whether the misfire is truly in cylinder 6 or if it's being caused by another underlying mechanical problem? Any insights from owners of similar models would be greatly appreciated.

Related fault codes

P0306

Comments (2)

Anonymous 2016-01-20 08:14

The spark plug replacement is a good first step — but ensure the plug is actually firing. Check its appearance: wet, dry, black, or tan? A dry or dark plug may indicate poor combustion. Next, verify that the fuel injector for cylinder #6 is pulsing properly and isn't leaking. If it's not pulsing, that could be a root cause. After confirming injector function, inspect for vacuum leaks and conduct a compression test on all cylinders. Using an OBD2 scanner to monitor live data — including fuel trim values and oxygen sensor readings — can provide additional clues about the misfire origin.

Anonymous 2019-05-27 18:09

This is a well-documented issue with the 5.3L engine, especially in older models like my 2009 GMC Avalanche LTZ. I'm currently troubleshooting this exact problem and sharing my findings as they develop. Symptoms include: - Stabilitrack and traction control disabled - Check Engine Light flashing (may stay solid when idle) - Rough idle and rough running at speed - Hard gear shifts, especially during downshifts In most cases, the issue stems from either a misfire or a stuck lifter in cylinder #6. Many users initially focus on ignition components (plugs, wires, coils), but these fixes often don't resolve the problem. My best guess is that removing the valve cover and inspecting for a stuck lifter could fix it — this has resolved similar cases. Other solutions are often overly expensive: trading in the truck (~$3,000) or replacing the engine (~$4,000). About my truck: - OBD2 codes: P0306 (cylinder 6 misfire), U0073 (CAN bus communication failure), and P0700 (transmission control malfunction) - Onset: While driving at ~15 mph, the CEL lit up, followed by Stabilitrack and traction control warnings. I checked with my OBD2 scanner and confirmed P0306. - After a 5-minute drive, no improvement — codes remain, including U0073 and P0700. P0306 confirms misfire in cylinder #6. P0700 suggests transmission control issues — possibly limp mode during shifts. U0073 indicates CAN bus 'A' communication failure between modules. I don't fully understand this code, but it may relate to ECM not communicating with the scan tool or other systems. Question: If U0073 appears, should I check if that's the only code when I next test? Is this a known GM issue? Should I contact the dealer — though my truck is 11 years old and service costs may be high? Am I on the right path with diagnostics? Any suggestions or real-world experiences would be helpful. I've previously received misleading advice (e.g., removing A/C improves HP on a 97hp engine), so I'm cautious. ~Myke~