P0305 Quick Fix Guide
A cylinder‑5 misfire can feel like a hiccup in the road, but most of the culprits are cheap, easy‑to‑replace parts. Before you book a shop appointment, run through this checklist – you’ll often pinpoint the problem in under an hour and save a few hundred dollars.
Check the Basics First
- Spark plug condition – Remove the plug from cylinder 5 and inspect the tip. Look for carbon fouling, oil deposits, or a cracked insulator. If the gap is out of spec (usually 0.030–0.040 in for most modern engines), reset it with a feeler gauge. A worn or fouled plug is the most common cause of a single‑cylinder misfire.
- Ignition coil health – Swap the coil (or coil pack) from cylinder 5 with the one on cylinder 4. Clear the code and take the car for a short drive. If the misfire follows the coil, you’ve found the faulty unit. Many OEM coils fail intermittently, so a “swap‑test” is more reliable than a voltage reading on a bench.
- Vacuum leaks – A small leak near the intake runner for cylinder 5 can lean out the mixture enough to cause a miss. Spray a little carb‑cleaner around the throttle body, intake manifold gasket, and any vacuum hoses while the engine idles. A sudden rise in idle speed indicates a leak that needs sealing.
Target the Ignition System
If the plug and coil check out, move on to the wiring.
- Connector pins: Disconnect the coil connector and look for corrosion, bent pins, or broken clips. Clean with contact cleaner and reseat firmly.
- PCM signal: With a scan tool, monitor the coil’s primary resistance while cranking. A sudden jump or drop compared to the other cylinders points to a PCM output problem, which may require a professional re‑flash or replacement.
Verify Fuel Delivery
Even a perfect spark can’t burn if the injector isn’t spraying.
- Injector test: Listen with a mechanic’s stethoscope; a clicking sound means the injector is pulsing. If it’s silent, try a 12‑V test – connect a fused 12‑V lead to the injector’s plug for a few seconds; the engine should stumble as fuel floods cylinder 5.
- Fuel pressure: Low system pressure can starve a single injector, especially on high‑pressure common‑rail setups. Use a gauge to confirm the pressure matches the manufacturer’s spec (usually 40–60 psi).
For a deeper dive into each step, see the in-depth walkthrough.
Takeaway: Most P0305 codes resolve by swapping the spark plug and coil first – if the misfire stays, follow the wiring and fuel checks to isolate the exact fault.
