My 2001 Dodge Caravan started running poorly after I got gas and the check engine light came on soon after. The OBD codes are P0302 and P0420. I have changed all the spark plugs and plug wires but it still has the problem. Would someone please let me know what steps I need to take next to diagnose the problem and ultimately fix it.

 

step 1, get a factory service manual this will have the step-by-step procedures to how to diagnose/test/and ultimately repair the problems
step 2, get a real scanner so you can read data of what is causing the codes.
step 3, google search the codes to get info on what they mean. like I just checked for you and a p0420 is a catalyst failure. now you'd need to use testing procedures laid out in the FSM to determine the failure and how to correct. P0302 is a misfire on cyl. #2. so maybe you have a bad wire, bad valve, bad injector, bad rings, bad head gasket, or something like that. that's where a shop manual and scanner can pinpoint the exact problem

 

I'd like to see that fuel sample.......

 

what fuel sample?

 

Bad fuel will cause the P0420 code too. Gas mix with Diesel? its always good ideal to get a fuel sample.

 

I've seen E-85 do it in a non FFV too.