qI just went thru the inspection problem with my 2004 Pontiac Bonneville with a 3.8L in NY state.You can have one error code as long as MIL light is not on. Refused to be held hostage by a repair shop. A ton of info on internet using google. Your PO300 code is multiple misfire. PO304 would be the only the number 4 cylinder. I think you mentioned cap and wires. Everbody gets excited about the vent screens. But unless their plugged, or their is moisture under cap I just do not see the problem. I would go with maybe cap, wires(can be tested), and plugs. Then like another poster suggested drive it a while. I found also that one problem can start a cascade of false readings. But, if that is not your case, PO420,PO430 is the catalytic converter. Probably most catalytic converters can be saved. They read underperforming or something to that effect.The catalytic converter will throw a code when it is only 4 to 6 percent plugged. I have put a gallon of paint solvent in the tank and driven for about 150 to 200 miles. I have used lacquer thinner. One other time on another car I took the catalyic converter off and soaked it dish detergent and water for a couple days then rinse with water. For the oxygen sensor test with a high impedance meter like an old Simpson 260 multimeter off ebay(about $65). The readings for O2 sensors can be found on the internet. If you need a new one, I usually buy the Bosch Brand. You usually can cross reference a GM number to a Bosch number. They are 1/4 to 1/2 price and probably better. Manufactured in Japan and not from the country of almost right. It seems like there is a logical sequence to the MIL scanner readings. In your case if the engine was misfiring unburned fuel went out thru the catalytic converter and fouled it. Might have even crudded up the O2 sensor. I would stop what your doing now, do not get so excited, read enough until your feel you have a little knowledge about the system. Know just a little bit about how the ecm and pcm and other sensors do on the truck. By a $50 scanner that tells you the code and the problem. I will like to bet that more than 75% of the parts the so called mechanics put in vehicles are unneeded. I like working on tractors. The only cars and trucks I work on are my own and a few friends. Also, make sure you oil has been changed not to many miles ago. If the vehickle burns oil I put in 20% lucas Oil treatment and 80% motor oil. I thouht it was snake oil until I used in two higher price tractor engines. Also, if you get it where the MIl does not come on you can do the GM method on how to start and the driving procedure(takes about 15 minutes). I am jumping around with different ideas. Also it helps if you have 100 miles on it with no MIl before the gm test. Also, scan IM for 6 oks. You should have you gas tank between 1/3 and 2/3 full(helps you EVAP system work better. One of the biggest problems I see is most guys are scared to get grease and dirt on their hands. And, do not want to learn how to use a good multimeter(the book that comes with a Simpson is about ten little pages). If you fall into either of these catergories take it to a repair shop. Most vehicles(quite a few now) I usually spend between 25 to 100 dollars on. Also, remember you are working on a truck and not the space shuttle. One last thing nothing to do with your truck. I do a little commodity trading(40 years and a hazard I got being in farming). Our Russian friend Vladimir Putin(a lawyer by education) controls thru himself and his companies 40 percent of the worlds supply of palladium which is the active catalyst in almost all catalytic converters. So,I hope Obama does not sanction him to much or the price of palladium(converters) could easily double.
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