Check engine light came on for a Toyota Prius that is only being driven about once a month since COVID-19 pandemic began.

OBD-II scanner shows P0302, which indicates the problem is a misfiring cylinder (cylinder #2, if it matters).

Vehicle had no issues before today, except:

  1. Possible minor hesitation when pressing the gas. Usually not noticeable, but occasionally felt some lag.
  2. When started 2 months ago, the vehicle made some horrible sounds shortly after starting. After about 5-10 seconds, the sounds stopped, and everything worked normally. Those sounds have not recurred since then.

Since that drive 2 months ago, there have been no audible or visible problems at all (except the check engine light coming on just now).

Spark plugs were all changed at 90K, and vehicle has 150K on it now. Those plugs should last for 150K (until 240K), so they should be okay. Putting in new plugs is a bit of a chore on a Prius (here is a good instructional video, if your are interested).

I don't know anything about misfiring cylinders, so I did some research and found much conflicting advice.

Is this something that should be fixed before driving the vehicle at all? Or is it likely something that is just the result of lack of use and can be resolved by just going for a long drive with some fresh fuel and maybe an additive?

If it needs to be fixed before driving, is it recommended to drive it to a mechanic (15-25 minutes drive each way), or have it towed / hire a mobile mechanic?

And yes, it needs to be driven more often. With so much going on related to COVID-19, there just hasn't been time nor any reason to go anywhere.

  • Its never a good idea to drive it with a misfire.
    – Moab
    Jun 27, 2020 at 2:34
  • 1
    Pro of misfire? I guess the pro would be you'd be getting a new car soon?
    – narkeleptk
    Jun 27, 2020 at 3:58
  • @narkeleptk LOL! I'm thinking more along the lines of driving it possibly correcting the issue, similar to an Italian tuneup. But if you're buying... ;) Jun 27, 2020 at 4:14
  • Cheaper to fix the misfire than replace the catalytic converter - excess fuel can damage them.
    – Solar Mike
    Jun 27, 2020 at 6:47
  • @SolarMike Thanks Mike. Especially true on a Prius where the cat is integrated into the entire pipe. But how much driving would it take to actually damage the cat in a little 4-banger like the Prius? 5 minutes? 10 minutes? An hour? 10 hours? More? Jun 27, 2020 at 12:34

1 Answer 1


I can relate a story from my own experience with my daughter's car. Turns out it had two bad ignition coils that caused the engine to misfire when it was raining. Did she stop when the check engine light came on? Nope!

So the unburned fuel from the misfiring cylinders ends up in the catalytic converter. These things are designed to handle mostly burned combustion gasses, not raw fuel. So they get hot, REALLY HOT! Hot enough to cause internal damage to the converter and increase exhaust back pressure significantly. This caused hot combustion gasses to be forced through one of the tubes that carries a bit of exhaust into the intake manifold, which is plastic, and which caused a hole to melt in the manifold.

So NO, you should NOT drive a vehicle with a misfiring cylinder. At most you should drive it a short distance to a shop. Other than that, have it towed.

A real mess. To fix required a new catalytic converter, a new intake manifold, several pipes/tubes which were damaged, as well as the two ignition coils. It turned a $100 repair into a $2000 repair!

  • Confirmed my comment then.
    – Solar Mike
    Jun 27, 2020 at 11:54
  • Thanks jwh. Why is the unburned fuel hotter than the combustion gasses? Also, do you know about how many hours it was driven before this problem happened? Jun 27, 2020 at 12:37
  • If the misfire is due to a faulty injector providing a weak mixture then a different set of problems may occur including melted pistons.
    – HandyHowie
    Jun 27, 2020 at 16:09
  • @RockPaperLizard A catalytic converter is designed to catalyze unburned fuel in the exhaust flow. It's sized to handle the amount of fuel left in a normally functioning engine. It can, however, be overwhelmed by excess fuel. Basically that's what a cat does.
    – jwh20
    Jun 27, 2020 at 21:56

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