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172 O2 sensor lean code


jballard81

Terminal Procrastinater
Supporting Member
Joined
Feb 24, 2024
Messages
517
City
North Carolina
Vehicle Year
1994
Engine
4.0 V6
Transmission
Automatic
Replaced my fuel injectors and many of the old vacuum hoses recently.
Afterwards I'm getting code 172 which is indicative of a vacuum leak. (I just replaced both O2 sensors)

With engine on I went around and sprayed all my hoses and where the intake meets with carb cleaner and didn't hear any engine changes.

Kept digging around where I was working and found that this tiny plastic hard line was broken
1000013369.jpg

That T leads over to the air box so that could account for the leak (maybe?).

I'm going to cap the T for now, but what is that little reservoir looking thing on top of the AC box? I need to know what it does so that I can prioritize it's replacement 😂

As always, thanks!
 
Last edited:
So capped off that T and went for a ride. Check engine light came on again, after about 5ish minutes of drive time, same code.

In terms of trouble shooting, I have a vacuum gauge I believe. I've never used it before but it makes sense to me to figure that out and if it's within the correct range maybe that means my O2 sensor already went bad?
 
Found this thread with a similar sounding issue:

I can check the intake and exhaust torq. Ultimately I can redo the intake work and check everything again. Would prefer to not do that if possible.
 
Went through all the vacuum hoses again, no real changes but drove it again and didn't get a code. Went for a longer drive and it through a code for just a second and then went away.

Maybe the gasket is settling in or the PCM is relearning or who knows but it seems the code is going away.
 
I have the same intermittent code. I highly suspect mine is from an exhaust leak at the flange to the catalytic converter. It only happens when I idle or drive with low rpm for several minutes. I didn't have problems until I cut one of the bolts out of that flange.
 
I have the same intermittent code. I highly suspect mine is from an exhaust leak at the flange to the catalytic converter. It only happens when I idle or drive with low rpm for several minutes. I didn't have problems until I cut one of the bolts out of that flange.
I'm going to rig up a smoke tester today I think. I haven't messed with the exhaust of this truck at all, but i wouldn't be surprised given the neglected state that I found it that it could have an exhaust leak
 

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