'01 Sierra Throwing Multiple Codes

[jdaniel83]

My '01 5.3L Z71's SES light came on about a month or so ago, but the light went off the next day and the truck ran fine so I kept driving it. About two weeks later it came back on and the next day it was back off and once again it was running just fine. Hooked my scanner up to it and it showed a code for one of the O2 sensors (P0155 to be exact) and a lean condition (P0171 if memory serves me correctly) but since it was running good I cleared the codes to see if they would come back. Well after it did it again today (SES on and then back off about 50 miles later) I hooked up the scanner but this time I not only got the P0155 code I got P0135, P0141, P0171, and P0174. Truck is running fine but the intermittant SES light is bugging me.

My scanner lists possible fixes as new O2 sensors, replacing intake manifold gasket, or replacement of cat converter. The truck has 199K and the cats are the originals, O2 sensors replaced about 100K ago, and intake manifold gasket was replaced a few months ago when I was having knock sensor issues. Any suggestions on what to look at first or is there any way to tell what the culprit is before I start throwing money at it somewhat blindly?

[ttopcamaro86]

I would check for a possible vacuum leak or cracked intake. Sounds to me like unmetered air is getting in somewhere. (I am not an expert though)

[jdaniel83]

Thanks for the reply. I could see the possibility of a cracked intake because I've taken off and installed the intake about 5 times in the past because of knock sensor issues so I could have accidently cracked the intake somewhere along the way. I've always torqued the bolts down in a two-stage sequence as per the Haynes/Chilton manual suggests, but that doesn't mean that the intake didn't get cracked somehow.