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The “reset and try again” diagnosis method isn't a good idea for most trouble codes, but in the case of a Po30# code, it's not a bad idea. Often times this code can merely indicate bad fuel...over-oxygenated gasoline may have entered some cylinders and caused multiple fires, which affects timing temporarily and causes problems. If the vehicle runs well after resetting the codes, this is almost assuredly what has happened.

In many Cadillac models, especially 4- and 6-cylinder cars, this is not unusual when the wrong grade of gasoline was used or ethanol is included in the mix unexpectedly or at higher concentrations than the car is rated to handle (such as E15 in an older GM vehicle rated for E10 or lower).

If, however, the P030# code comes back – or if there are symptoms of a random misfire problem (stalling, sluggish acceleration, rough idle) – there are several things you can check.
  1. Begin by checking all spark plugs and their wiring for clogged, broken, or excessively fouled plugs. Considering the low cost of spark plugs, it's not a bad idea to just go ahead and replace all plugs at this point in your diagnosis.
  2. Check for unplugged or resistive wiring (older spark plug wires can deteriorate and diminsh spark).
  3. The coil pack, oxygen sensors, fuel injectors, and exhaust valve(s) should also be checked.
  4. A stuck exhaust gas recirculation (EGR) valve could also be to blame, as could a faulty camshaft position sensor.

The good news is that a P0300 code is almost never indicative of a serious problem. Still, this code should be addressed quickly. If you deal with it right away (and it's typically inexpensive and relatively simple to take care of), you'll likely enjoy better fuel economy and performance, not to mention no more “check engine” warning lights.

Author Jason Lancaster is the editor of AccurateAutoAdvice.com, a website that helps consumers find the best information when it's time to buy, sell, or maintain their vehicle. Jason also works with GMPartsOnline.net, a company that sells OEM Cadillac parts online at discounted prices. See their website for more info.