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Discussion Starter #1 (Edited)
In May, I hit 5 years/60K and decided to buy my 06 Cayman S off lease. Co-indecently (or by design), while driving home from dinner last week, the CEL came on. There were no other symtoms or extream conditions. When my tuner shop connected in, we learned the following engine error codes: 421 (Efficiency..?) 301 (Misfire in Bank 1..?). After conferring, the two techs suggested I replace the plugs and coils. They only removed the CEL and it has stayed off since, awaiting my decision. Otherwise, no other symtoms/noises.

Does anyone have any experience with these codes and their meaning? What may have caused it? After 60K the plugs are due. Interestingly, the techs feel that running a flashed ECU may have caused the ignition coils to retire early. Where is the best place to purchase replacements? Stay with OEM Bosch? I am aware of updated coil packs. Are they the best bet?

Other background intel: In May of last year the dealer replaced the left cat due to error code 431.

Thanks for your thoughts,
-Stevie
 

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A search here will turn up what to do about the 421 and 431 cat efficiency codes. What should have been done was to replace the factory exhaust manifolds with the updated versions from Porsche and flash your ECU with the lastest Porsche flash. That would have fixed both of those codes.

As for the misfire code, it's not uncommon, even in normal operation, to get some misfires, but also if you've ever tracked your car and ingested oil you will get misfires registered. Unless the coils or plugs tested bad there was no need to replace them unless you simply wanted to.

I would recommend clearing the misfire code and see if it comes back and if it does, how often.
 
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