CEL light keeps coming back on...HELP!

[climbinglife]

Ok, here is the story so far...any help or insight would be appreciated. 03' SE w/57K on the clock.

About 2 weeks ago, my Service Engine Soon light came on. I bought it in to my mechanic who i've been going to for about 10 years. they are good guys who have always treated me well and have done good work. He pulled the code, which i think he said was #01704, but I'm not really sure at this point because he didn't write the code on my receipt. He said it was a problem with the MAF and said I needed a new one. He said the OEM was crazy expensive and quoted me a price of $380 for an aftermarket. He installed it and did the computer re-learn and off I went. 2 days after that, the light came back on. I put less than 50 miles on it. He checked the car and found one of the hoses near the intake/throttle body had come off and assumed that was the problem. He re installed the hose, cleared the code and off I went. I filled the car up and took a trip upstate to see my parents...about a 300 mile ride.

I noticed my gas mileage had gone in the toilet. I'm used to getting 14.5-15.0 city and 19.5-20.0 on the highway. I got like 14 on the highway and my next tank was all city and I got like 10.5-11.0. Midway through the second tank, the light came on again. Brought it back and he hooked the computer up and now said he could no longer read the MAF, like it wasn't producing a code. Said it must be defective so he ordered a new one. They put the new one in and did a re-learn again and off I went. 50 miles later, which is today, the light came back on again. I called him and explained and I'm bringing the car back in the morning to leave with him to figure out whats wrong.

The MAF cost me $560 total with a two year warranty on the part so I haven't paid anything else after the initial one but now I'm concerned that the MAF wasn't bad all along and something else caused the light to go on initially. When i see him tomorrow i;m gonna ask specifically what the first code was that he pulled. The care DID NOT act any different at all, except for the gas mileage tanking. I find it hard to believe that 2 MAF's aftermarket or not were defective so i'm confused.

Anyone have any insight or other possible things it could be? I know not having the first code doesn't help, but could it be that I need an OEM MAF?

I LIVE in this car. I use it for work and school and I'm getting a little bummed, feeling like I might have made a mistake buying this car

[Towncivilian]

I wished you had come here instead of to your mechanic, because an OEM Maxima MAF that's $89 can be used without issue over the $442 Pathfinder MAF.

If possible, please get codes pulled at Advance Auto Parts or another store that reads codes for free and post them here. Without codes, we'd just be shooting in the dark to try to find a cause.

Welcome to NICO!

[climbinglife]

Really? ugh, that sucks. Would have never figured that. I'm gonna have him write down any code he pulls and will update the thread when I have it.

[Densetsu]

Was the code P0174? If so, it could be a number of things. A dirty or faulty MAF is very common, as is a vacuum leak. Could also be a bad pre-cat O2 sensor (driver side) or a clogged fuel filter. Sounds like the mechanic has tried to address the first two possibilities.

If it IS P0174 (System too lean - bank 2) and there's no accompanying P0175 code (System too lean - bank 1), I'd suspect it's caused by a component on the driver side. There's a simple way to narrow things down. When you go back, ask for him to check the freeze frame data when he pulls the code. Specifically, look at the long-term fuel trim for both banks when the code was thrown.

In my case, the long-term fuel trim for bank 2 was around +12%, and for bank 1 was +2%. (Note: Ideally both figures should be at zero.) That's a large discrepancy, which definitely points to an issue specific to bank 2 (driver side). I replaced the driver-side O2 sensor and the light never came back.

If the two fuel trim numbers are fairly close, like +9% and +11%, then it's most likely caused a component that both banks use -- MAF, vac leak, fuel filter, etc. It may be that both trim numbers are high, but only one is high enough to throw a code for that bank. Note that this scenario may also occur when components on both sides went bad at the same time, e.g. both O2 sensors are bad.

Freeze frame data is your friend. If you can get that to us as well, post it too. Not just fuel trim, but everything.

[climbinglife]

I really wish i would have came here sooner...it WAS the Pre CAT 02 Sensor AND the CAT itself had a hole in it. There still telling me that MY MAF was bad to begin with and the first one they replaced that with was defective...oh well, live and learn. Needless to say with the amount of money I've put into this car since I bought it two months ago, nothing else is getting done unless the wheels fall off, lol.

Thanks for everyone's help, it's appreciated.

Regards,

Vin

[zach7685]

Sound like the maf was bad and it ran rich dumping unbured fuel into the Cat which caused a failure