MinnesotaBum

I have a 1999 chev silverado 5.3 with the service engine light on. I pulled 3 codes out a P0171, P0172 and P0420. My local service station told me to replace the two O2 sensors in front of the cat, that did nothing to help the problem(light still on and doesn't want to idle smooth) Then they told me to replace the two behind the cat. I dont want to keep throwing money at this in hopes it will get fixed, anyone have any ideas here, I will replace the other two O2 sensors if that will fix the problem, I just want to be sure. I have given it a tune up(only AC delco parts) and checked for vacuum leaks. What else should I try?


Thanks, A MinnesotaBum!

lukemoore646

test the fuel pressure regulator for a leak, if you take the vacuum line off of it does it smell like gas in the line?

MDTAHOE

Replacing the rear sensors will NOT fix it. It is possible that you have failed intake gaskets. Common problem with the 5.3, gasket has actually been updated. PO 171 is lean bank 1 and PO 172 is rich bank 1. What maybe happening is the intakes leak when the engine is cool so the PCM sees a lean condition and begins dumping extra fuel as the truck warms up the intake seals back up it is no longer lean but the PCM has been dumping fuel to keep up now there is no extra air so it is rich (temporarily). PO 420 is catalyst efficiency Bank 1 99% of the time it is a bad converter. However with the other problem occurring it could be set because the O2 sensor signal is all over the place. You need to take care of PO 171 and 172 first. If po 420 comes back then you will need a converter. 99% of the time O2 sensor codes are not a bad sensor. The code just means the O2 sensor is seeing the problem. Also on a OBD II vehicle O2 sensors usually will not cause it to run bad because the PCM sees the problem and uses another strategy to control fuel.

lukemoore646

and that is why he is a servise manager and i am just a wrench turner

MinnesotaBum

Ok, thank you. But I have made a mistake in my posting, the P0172 code should have been a P0174 which is lean bank 2. So the three codes are P0171, P0174, and P0420. Does this still point to a possible intake gasket leak? What is the best way to test for a leaking intake gasket? Any help is appreciated.

MDTAHOE

Absoultly, But you may want to try to clean the mass air flow sensor first.

lukemoore646

i think the spraying carb cleaner around the intake gasket area at an idle trick should work on this

MinnesotaBum

Thank You so much for the help. I have replaced the intake gasket and it now runs like a top.


Thank You!

MinnesotaBum

Garyrstevens

My 99 silverado has a bunch of codes on it also. P0449(evap). P0352(coil2). P0300. P0171(o2 sensor). The truck starts great on days, then on other days it starts rough, and then on first take off it has no power at all, missing and hardly not going at all, then a mile or so it runs out fine. Seems like it does it when it gets warm outside. Motor has 209,000 on it. It sounds like there is a vacuum leak under the hood, but I cannot pin point it. I sprayed carb cleaner around intake, but no changes. My gut tells me it is the intake gasket. Anyone having this problem. I did just recently change the fuel pump because my gauge was not working, so I changed it all. But it did this before I installed new pump also

Trevor Thompson

Not sure what the codes are. But it might be a blown head gasket allowing moisture in during the night and if you start it up and it has a lot of white smoke then it's a good sign it's the head gasket