Hello Scotty,
I have a nissan sentra SR 2011, automatic transmission, and around 180k miles. Engine shaky and runs rough past the 40mph. Put the scanner and got the codes mentioned above. Changed oxygen sensor, sparkplugs, coils, and tested the fuel injectors with multimeter for resistance and all 4 are within the same rage, 12.9-13.2. problem persists. Could you give me a clue where to look next?
Thank you
A quick online search brings up information on possible causes of those codes:
https://www.obd-codes.com/p0420
https://www.obd-codes.com/p0303
https://www.obd-codes.com/p0304
Anything interfering with combustion can cause misfires, including internal engine problems such as burned valves. Checking resistance of the injectors tells you nothing about whether they are clogged or leaking, or whether you have sufficient fuel pressure. More diagnosis is needed before throwing parts at the problem. See the FAQ for more details.
Did you use a OEM o2 or did you use a universal or cheap generic. Also what could did you use. Just because the injectors ohms are is in range it doesn't mean it good. That only lets you know the electrical resistance between 2 points. You still have the actuation of the injector itself and the continuity. I'm thinking your cat is bad. Doing a pressure test is the most accurate way but there are a couple other things. With the car running put your hand at the exhaust tip. You should easily feel the exhaust flow. Get a thermometer and check the temp of the cat. You want to start car and let it get to running temp you want to check the very front, center and rear . The temp should be the same or really close. If there is a notable difference of your cat is likely clogged.
