Q: Code PO300 and PO430
What cause these to set and why does this happen on 2005 Chevrolet Express 2500

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Towing a trailer about 6000 pounds at high way speeds
(2) Answers
(2) Comments
Have the random misfire (300) code fixed first that may take care of both codes. Mechanic with a 'scanner' is needed. Just for the heck of it, what engine? Might help a little with advise for first code. Second code (430) 'suggest' cat. converter insufficient operation. This may be due to the misfire.
Ok, on the V-8's here are some possibilitys, spark plugs, wires or 'coils', fuel injectors or fuel and vacuum leaks, scanner (not a code reader) will show which! Compression shouldn't be a problem, tech. with a good scanner can check cyl. misfire, injector balance, fuel trim and various sensors to determine your problem. This is, in my opinion, the only way to go! When was your last good tune up? Suggest AC Delco parts, if you plan to keep the van!
Good luck with it.
I agree with pushrod that the P0430 is probably due to the misfires. Random misfires can be caused by lots of things. How many miles are on the vehicle? Do you feel stumbles or misfires while driving? Does the check engine light flash when the misfires are happening? Please give more details.