I'm in the process of helping someone with their 2003 Honda Civic, 1.7L engine, 161k miles on the clock. Pulling the codes from the vehicle I got three specific codes: P0300, P0302, P0303.

The codes themselves equate to generic misfire codes, with P0300 being the generic root, P030x (where "x" is the final number) equates to a specific cylinder. Above, P0302 would equate to cylinder 2 and P0303 would be cylinder 3.

The vehicle owners took the car to a local muffler shop because the car was running rough at idle. The shop pulled the codes, replaced some parts, then suggested the fix could be to replace the headgasket. They also told them they weren't sure if replacing the headgasket would fix their issue.

What would be the proper diagnostic steps to determine what the root cause of these codes are?


2 Answers 2


Beyond doing the basics such as checking compression and visually confirming spark and fuel pressure (with a gauge). It's time to break out the scan tool and scope.

High mileage Honda's tend to suffer from the valves going out of adjustment. Take a look at MAP voltage either with a scan tool or by backprobing the MAP sensor signal pin. At operating temperature MAP voltage should be below 1v ideally around .8v. You could also visually confirm with a vacuum gauge off the brake booster hose. If map voltage is high and you have confirmation with a vacuum gauge you need to remove the valve cover and physically check each valve. Luckily Honda marks the cam gear so you know which cylinder is at TDC.

Another thing to check would be a short in the primary coil of each ignition coil. If you pull fuse 1 in the underdash fuse box and put in a loop you can attach a DC Inductive Amp Clamp and scope the current draw of all the ignition coils. It's best to get as high of a resolution as you can. I'll typically set the sweep time to 10ms.

A short in the primary coil looks like...

enter image description here

This isn't from a Honda but it's basically what you should expect. On the left the coil ramp is good. On the right you can see a sudden spike in current.

Regarding the head gasket I'd suggest doing a leakdown test. Honestly unless the car has a history of over heating and compression is good, this would be the second to last thing I'd check.

As for the last thing, It may be an injector, but I doubt it. Only on two cylinders and both at the same time? You could pull the rail and injectors and test the spray pattern and check the resistance of each injector. Or just swap them around to see if the misfire follows.

  • Would there be an even more basic place to start troubleshooting, especially for the average Joe? If I didn't have a way to pull the wave form, and even before I do a compression check? Commented Jan 12, 2017 at 17:51
  • @Pᴀᴜʟsᴛᴇʀ2 if your talking about ignition testing other than using a multimeter to check resistance or using an adjustable spark tester (to confirm strength vs spec.) or swapping coils around theres not much else you can do short of replacing parts. honestly though i'd lean towards the valves being out of adjustment. backprobe that map sensor and take a look at map signal voltage engine hot/running.
    – Ben
    Commented Jan 12, 2017 at 17:58

Generic misfire troubleshooting:

  • fuel: check the fuel pressure and electrically test the injectors
  • air: check the air filter for obstruction, MAF for cleanliness, IAC valve for cleanliness. When in doubt, clean them out, and don't forget any hoses leading from the IAC valve.
  • compression: a quick compression test is always a good idea
  • spark: inspect the spark plugs, test the plug cables for proper resistance, if equipped with a distributor, inspect the rotor and cap. Test for spark (I use a timing light).

In my experience, intermittent rough idle has been caused by:

  • cracked insulator on a spark plug that expands when hot
  • bad plug wires
  • worn out distributor cap and rotor
  • dirty IAC valve that sticks (I need to clean mine every 2 years or so on the Nissan)
  • I'm with you on the bad plug wire thing. Best is to pull each wire / system off of each plug and inspect it very closely. I've seen some plug wires melted right at the plug that weren't obvious until you pulled the wire off the plug and checked it closely. This makes the car idle rough.
    – zipzit
    Commented Dec 23, 2016 at 4:58

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .