As you may remember I was getting an intermittent CEL P0420 (catalyst system efficiency below threshold) code. Cleaned MAF, cleaned throttle body, changed plugs and coils. The code disappeared and I thought I nailed it.

Yeah right! Now the game changed! This morning (2 months and 2K miles later) I've got P0300, P0306 and P0303 (multiple cylinder misfire code). CEL just lit up - not flashing. I've cleared the code and all good again.

Engine runs perfectly, no stuttering or irregular idle. Should I ignore or do you guys have any troubleshooting ideas?

My petrol was on nearly empty and the car was sitting about for two weeks. I will eliminate bad fuel first. I will refill and use injector cleaner albeit I doubt this will be it!

Any ideas are welcome...

Thanks guys.

 

Could be ignition coils, especially if it has been raining where you are lately. But with multiple cylinders misfiring, it is probably something common to all cylinders (for example, as you mentioned, bad fuel).

 

Have never had a P0420/P0430 error code with my Turbo. My Boxster produced a P0430 error code off and on every once in a while over a couple of years. The underlying cause was a loose converter brick.

One could hear it knocking upon cold start and after everything got warm it buzzed something fierce.

Finally got tired of it and bought 2 used exhaust manifolds in which the converter is part of the manifold and had these installed. Noise gone. And P0430 has never come back.

The factory guidelines for a P0420 is if the error is accompanied by any aging O2 sensor codes to replace the indicated sensors and clear the codes then road test the vehicle. If the P0420 comes back replace the indicated converter.

For the misfires, I've been dealing with the same codes now for a couple of years.

The misfires only occur upon cold start within less than a minute after a cold start. Just about the time I'm thinking " the engine is idling a bit rougher than usual" the CEL comes on and sure enough it is same 3 codes: P0303, P0306 and P0300.

The O2 sensors were replaced at 132K miles. Didn't help the misfires although the engine ran a bit better.

I checked with the tech and we decided to replace the original coils, this at around the 140K mile mark. At the same time he recommended new plugs even though the plugs were about half way through the 24K mile (or whatever it is) change schedule. I agreed.

While the engine ran a bit better after new coils and plugs -- and I've had the plugs changed before with no change in the engine so I attribute the improvement to the new coils -- the misfires were still around.

Might state here the engine never misfires are in any way misbehaves at any other times.

The tech offered the opinion sticky injectors and gave me a bottle of Swepco fuel system cleaner. I tried this and it made no difference.

I then tried a bottle of Techron. Treats 20 gallons but I dumped the entire bottle into the fuel tank then filled it up. Ran the tank down to around 1/4 level and added a 2nd bottle and then filled up the tank again.

No real improvement in the engine and the misfires persisted.

While Techron didn't help in my case I'd suggest you give Techron a try. Run a bottle and regardless if you notice any difference run a 2nd bottle.

BTW, after using Techron it is advisable to change the oil and filter. Techron results in extra oil contamination and these engines do not need any additional contamination.

Next time I take my car in for service I'm going to speak to the tech about what he recommends I do next.

The misfires are more a nuisance than anything else in that I have to disconnect a logging device and connect an OBD2 code reader then read the codes and then clear them then reconnect the logging device.

But I found a way to avoid the misfires and that is instead of my usual cold start procedure where after a cold start I let the engine idle until the secondary air injection pump shuts down and the idle speed drops to near normal hot idle speed I just wait a moment or two then ease the car out the parking space and then drive it easy until the engine is fully up to temperature. IOWs, no letting the engine idle a bit before heading out.