2002 Silverado throwing codes (Page 1/1) Hank is Here AUG 16, 10:23 PM I have. 2002 Silverado 4.3. W Vin 2wd 175k miles, 4300 hours,and it is throwing two codes, one of the codes has been hitting often enough to cause the SES to flash. The first code is P304; the truck has thrown this intermittently the past 100k or so. The light may stay on for a day to a week. The code means misfire on cylinder 4. The recent code is P420, which means catalyst is below efficiency. This code has started popping up the past week or so and causes the SES to flash when I am under moderate to heavy acceleration. The cat is not ratteling, fuel mileage is normal. It does seem I have to use more throttle under load than normal. How can I diagnose the problem without throwing new parts on it? A new cat is $375 andfour O 2 sensors are about another $100 or so. The is my dd vehicle. Given the choice of fixing fast cheap or correct I'll opt for correct and cheap rather than fast. Ideas and feedback welcome. joesfiero AUG 16, 11:06 PM You need to diagnose the 304 first because that (a misfire) can cause the 420. Start with basics, pull the plugs and look carefully at them especially Number 4 and see what it tells you ( running lean, covered in oil, etc.) or if the plug itself Is at fault. If you want, throw a pic of them up here and let us look if youre not sure what you're looking at. -Joe Raydar AUG 16, 11:22 PM The newer OBD2 diagnostics are really good. Strangely enough. I had a cat efficiency code (P0420) to pop up on our Miata. Replacing the cat fixed it. I really expected it would be more difficult, but it wasn't. But yeah. The misfire can dump raw gas into the exhaust. It can either kill the cat, outirght, if it's bad enough (cats hate raw gas) or can at least cause some raw gas to get through, "unpprocessed", which the downstream O2 would pick up. I would NOT replace the O2 sensors. Sounds like they're doing their job. Hank is Here AUG 28, 04:53 PM A little update on the codes here.... After the code 304 and 402 I decided to put in new spark plug wires, cap and rotor since I had thes items laying around. After replacing everything the codes cleared however the truck started missing at anything above idel. I then got a new set of plugs to see it that was the cause of the miss, the problem continued. I tried disconnecting amd reconnecting the wires from the plugs and cap with no luck. I also did a detailed check for any disconnected or broekn sensors under the hood, again no luck. I needed to drive the truck 200 mile sone day last week with the missing it was very tough. I was leaning towards the cat being plugged cuasing the mis and low power. Yesterday I was going to drop the cat to see if it was plugged. Before I dropped it I decided to pull the new cap and install the old cap. After putting on the old cap the truck ran great! I cleared the codes stored on the computer. Between last night and today and all is well, no codes have re-appeared. I guess the replacement cap was bad from the factors. As close as I can figure is that the conneciton from where the coil attaches to the center of the cap there must be a bad connection inside of the plastic. The bad cap is an Wells brand. Generally I thought Wells was a good OE brand, but no more. Looks like I'll be buying Standard parts from now on.