Thread: P0300 & p0420

Hey guys! SES light came one about 2 hours ago. Got it scanned and P0300 & P0420 codes came up. About a month ago my car was running horrible; engine was hesitating, very little power, and stuttered. No SES at the time. Changed the MAF, Plugs and wires, fuel pump and all did nothing to help. LAst weekend i gutted the cat and rigged my o2 sensors with two defoulers. Car ran better then it ever has before. Power was awesome and got to 95 mph with ease, keep in mind i had never gotten past 75 in the last year. About 150 miles after i gutted the cat the codes above came on. Also, i was getting 12 miles/gallon at my last fill up. Any suggestions to what my problem is?

blueGTP01, Yeah my car is an 02'. There is a place close to me that does tuning but they will only tune O2 sensors to non-report for off-road vehicles. Don't know a way around it. I was starting to think the defoulers were going to work after 150 miles but they failed me. As for the P0300 i will check everything you said. Any ideas from anyone else would be appreciated too. Thanks blueGTP01.

That is prob to best solution. However, you said if i "DD" the car. What do you mean by that? Also i heard that sometimes when you use a HFC, the O2 sensor still sends a code to the PCM. IF that is the case i would still have to tune my PCM. But what is DD?

Originally Posted by demetriuslopez

Hey guys! SES light came one about 2 hours ago. Got it scanned and P0300 & P0420 codes came up. About a month ago my car was running horrible; engine was hesitating, very little power, and stuttered. No SES at the time. Changed the MAF, Plugs and wires, fuel pump and all did nothing to help. LAst weekend i gutted the cat and rigged my o2 sensors with two defoulers. Car ran better then it ever has before. Power was awesome and got to 95 mph with ease, keep in mind i had never gotten past 75 in the last year. About 150 miles after i gutted the cat the codes above came on. Also, i was getting 12 miles/gallon at my last fill up. Any suggestions to what my problem is?

Hey there, have you changed the fuel filter? If so, do a pressure check on the fuel system. I hook pressure gauge to fuel rail and then tape dial to the windshield so you can see while you drive around the block, should not dip below 25-30 psi. Listen to the fuel injectors if you can, all should sound alike. Fuel delivery is based on about 4 sensors til the car gets to normal operating temperature, the it is solely based on the o2 sensor, which might explain the poor fuel milage. I'd put some sort of cat back on if it were me. Maybe that might help

Originally Posted by bluegtp91

Daily Driver. I didn't know if Indiana has emissions testing or not. Yes, there is still a possibility that a HFC will trip a CEL...chances ya gotta take. I'd have the PCM tuned to get rid of it...but you'll still need a rear o2 sensor.

Yeah i am a daily driver. I was looking on ebay for HFCs, but do i have to get one that is specific to a grand prix? Also, you said get the biggest one, but what if i still have the stock exhaust. How would I fit a bigger one to my stock exhaust?

Also for your PO300 check that none of the wires are on the wrong plugs. This kicked my ass for a couple days once.

i have a HFC, it didnt set off the light till like 3 months after i put it in, i dont get emissions testing so i dont care, but yea the downpipe is worth it