: Help with PC0174 code

03-02-05, 07:43 PM

I purchased a 1996 Seville SLS about 7 months ago. With the exception of a rough idle only when you first start it up it ran fine until about 2 months ago when it would not start. Turned out to be the positive battery cable that was badly corroded. Took it to the dealer and had the positive cable replaced, coolant flushed/changed and had the fuel rail replaced under the recall. Since then I have been getting a PC0174 code about twice a week and lately it seems even more frequently. I have also noticed it seems to idle very low, about 250 rpms (can't remember what it was before taking to dealer). I have the analog guage where it is at the first mark. When it is warm and I am in drive with my foot on the brake, it idles at about 200 rpm and the oil light occasionally will flicker. I have read in other posts where I think it should be about 450 to 500 rpms.
I have replaced the air filter, pvc value and cleaned the throttle body real good. I purchased the car from a small used car dealer that told me he had his mechanic replace the plugs and plug wires. I can not tell what brand the wires are, but the plugs are Autolite double platinums. After reading other posts on this board where you should only use AC Delco plugs and wires and that the wires can be damaged by the dealer after the fuel rail recall (thats probably if they are older), I have order a set of AC Delco wires and plugs.
Hopefully, they will be deliver by the weekend so I can put them on.

I looked up code PC0174 (Fuel Trim System Lean Bank 2) but do not know what to check for, or what needs to be replaced. I know I should probably take it back to the dealer, but I am afraid they will charge me a fortune that I don't have right now.

Any help would be greatly appreciated.


03-06-05, 01:42 PM

I replaced the plugs and wires yesterday with the correct AC Delco parts (plugs 41950 and wires 748J). Took the car for a ride last night and this morning and both times the PC0174 code appears again. I did not notice any difference in smoothness or performance with the new plugs and wires. The idle did not change and still seems to be low, about 250 rpms on the tach. After I replaced the plugs and wires I removed the negative battery cable for 30 minutes incase something needed to be reset.
Could the low idle be a bad ISC unit? I found another post where you can check the operation of the closed throttle switch with the onboard diagnostics. In the PCM mode the LO icon should be on if things are Ok. On mine the LO icon was not on, but the post was referring to a wandering and high idle. Since my problem is a low idle, I am not sure if a new ISC unit would fix the problem.

I founded some info on the internet that a PC0174 code can be cause by a dirty or faulty MAF sensor or a vacuum leak downstream from the MAF sensor. I checked for vacuum leaks but could find none.
Does anyone know how to check if the MAF sensor is bad? To clean it, should I just spray the sensor with the same stuff used to clean the throtle body or is there a special procedure to clean the MAF sensor?

I hate to just start replacing parts without knowing for sure they are bad.
Any help would really be appreciated.


01-02-08, 04:53 PM
I'm having the same issue with my 96 Deville. Anyone know how to fix it?

01-03-08, 03:06 AM
I read somewhere (I think in the Seville/Eldo forum) that one source for the P0171 and P0174 codes is either a disconnected or leaking/cracked vacuum hose. I think a good start is to do a thorough check of all the hosing.
I cleaned the EGR and throttle body on a 2002 STS and it started throwing those two codes. I haven't gotten back to it yet, so don't know if I left one off or what.
Let us know.

01-03-08, 11:29 AM
Yeah, look for a vacuum leak.

01-03-08, 07:53 PM
Don't spray anything on or in the MAF. In the '02 FSM P0174 sets due to fuel system lean/rich trim attempts. My guess is that, coupled with the low idle speed (600-650 spec), something is very dirty in and around the throttlebody and IAC system (Idle Air Control)

Surf around in here and do a search (top of page) for throttlebody cleaning and IAC cleaning. Don't use choke cleaner per the FSM.

Also find the thread concerning using Google to search inside these forums. There are differences between '96 and '02, so use this as a guideline, OK?