• The site migration is complete! Hopefully everything transferred properly from the multiple decades old software we were using before. If you notice any issues please let me know, thanks! Also, I'm still working on things like chatbox, etc so hopefully those will be working in the next week or two.

P0300 & p0420

D

demetriuslopez

New member
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?
 


P0300 is a random misfire...since you have new plugs/wires...might check the coils...or make sure you don't have a vacuum leak...check all the lines around the lower intake manifold...fuel rail...TB...EVAp...etc. A vacuum leak can cause a serious misfire. Spray some carb cleaner around the engine bay or listen for a hiss. If the carb cleaner hits a open spot, it'll throw your idle off. P0420 is low catalystic efficiency and that is from the cat you don't have. Just wondering what year your car is...if it is '01+ your going to need a tuner to remove the codes because cars after '01 have a different fueling loop and it'll kill your MPG if it doesn't have a rear o2 with the codes deleted or a converter in there.
 
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.
 
91.

Not 01.

LOL

Anywho...if I were you...I'd just get a High flow cat...it'll keep the car from running uber rich and you won't have to have it tuned. Save ya some money, especially since you can have the stock cat (whats left of it) and the u-bend taken out for less than a hundred bucks...and gain MPG's and power...and its perfectly street legal unlike your gutted cat...if you DD the car that is. YOu can get HFC's off of ebay in any size you want for like $50-$75.
 
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?
 
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.
 


demetriuslopez said:
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?
Click to expand...
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
 
bluegtp91 said:
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.
Click to expand...

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
 


You must log in or register to reply here.
Back
Top