Q: Flashing CEL

The codes you recorded, P0304, P0171, P0301, P0300, all pertain to engine misfiring. The flashing of the Check Engine Light signifies that the misfiring is "active" (ongoing) and consequently you should try to get the car repaired before operating it further and possibly damage the catalytic converter.

Ignition coils have to be tested before they are condemned. If you did not test them, it may be that the old one were perfectly good. It is best not to replace any parts until you either pinpoint the exact fault or work with a professional to get that diagnosis. Lots of times, perfectly good parts are replaced, without testing the parts, and that is needless nowadays with the availability of not only detailed Factory Service Manuals but mobile certified technicians such as those dispatched by YourMechanic.

Here is a list of the possible faults that a mechanic would check with the codes that you have:

• Spark plugs that are damaged or worn
• Spark plug wires and/or coils that are damaged or worn
• Distributor cap that is damaged or worn (only if applicable to the vehicle)
• Rotor button that is damaged or worn (only if applicable to the vehicle)
• Fuel injectors that are defective
• EGR valves or tubes that are clogged
• Ignition timing that is off
• Vacuum leaks
• Fuel pressure that is low
• Head gasket(s) that are leaking
• Camshaft sensor that is faulty
• Crankshaft sensor that is faulty
• Mass air flow sensor that is faulty
• Oxygen sensor that is faulty
• Throttle position sensor that is faulty
• Catalytic converter that is faulty
• PCM is faulty (rare)

If you seek a diagnostic, the technician from YourMechanic will walk you through a sequence and following is a partial list of some of the items that are relevant to the codes you recorded. Note, again, that any part that is identified has to be SEPARATELY diagnosed. That is, it is never wise to replace a part just because it is on a list of possibilities associated with a code.