Re: sho codes
P0301 and P0303 are both misfire codes- Cylinders 1 and 3 respectively. Given you have given P Codes you have a 1996-99 SHO. The PCM monitors engine speed and Upstream O2 sensor data to detect a misfire. Some basic things to check for a misfire is fuel pressure-too high or too low can cause a misfire, it should be 30-45 psi; check the spark plugs-check the gap, if there is carbon tracking (Dark grey or black lines on white porcelin insulator) replace the spark plugs. For carbon tracked plugs you must also replace the ignition coil for that cylinder because the coil will be carbon tracked too. For spark plug removal you may have to remove the upper intake to access the back spark plugs, which are numbered 1-4 from the R. side of the vehicle to the L. side. If these basic things are in order or they don't fix your problem than you must go deeper. Do a compression test first-disable the fuel sytem by disconnecting or hitting the inertia fuel shut off switch in the trunk, compression should be around 105-140 psi. Next, do a cylinder leak down test using a leak down or differential pressure leak down tester. You will have to hold the engine from turning when you do the test. Before the test crank the engine so the cylinder your testing is on top dead center of the compression stroke, then test the cylinder. Any leakage above 15% is bad- listen through the tailpipe, the intake, and the oil filler hole for leakage- Leakage through the intake means the intake valve face or seat is burned or has carbon build-up causing a leak, leakage through the exhaust means the exhaust valve face or seat is burned or has carbon build-up. If you have the engine on top dead center of the exhaust stroke you will have a massive amount of leakage through the exaust because the exhaust valve is open- just make sure the engine is at TDC of the compression stroke. If you have leakage through the oil filler than you have a problem with excessive blow-by which means the piston rings are not sealing the cylinder properly. But wait, there's more!!! If the leak down test indicates less than 15% leakage than one of the last things to check is the crankshaft position sensor and the tone wheel inside of the engine that the sensor reads.If the sensor does not have dirt or metal on it and the tone wheel does not have any obvious breakage than the last thing would be the PCM. I realize this seems like a whole lot for a misfire but this is what you have to do to find out if this is a $150 Tune-up or a $1000 engine overhaul or replacement. If you don't work on cars or you are not familiar with some of the things I have touched upon then take your car to a qualified ASE certified technician or to a dealer. Otherwise, Good Luck!!!