I have a 05 sieraa that's been sitting a few months...goint o use it as a plow truck...it has very rough idle and no power taking off....idles about 700 rpm...how do i do a coil test...wires seem fine...what else should i look for? Thanks

 

First off, the P0300 code indicates there is a nonspecific misfire and P0200 indicates the PCM has detected a malfunction in the fuel injector circuit.

I would say that the two codes are more than likely related and likely caused by something in the fuel injector wiring. The first thing I would do is to examine all the electrical connectors at the fuel injectors and make sure they are all plugged in and seated properly. Also look for any broken wires at the electrical connector for each injector. If the engine still runs rough after that, then get a noid light and plug it into each fuel injector electrical connector one at a time and make sure that the noid light lights up when you crank the engine. While you are checking with a noid light, you can also measure the resistance of each injector with a digital multi meter to see if the injectors are in spec. If you do not already own one, now would be a good time to purchase a Haynes Manual for your Sierra - it will prove to be invaluable in helping you maintain your Sierra.

Good luck!