1998 Yukon low operating temperature

[olliec420]

code...

 

[strutaeng]

code...

I'm pretty sure that means your catalytic converter is bad...

I think you can check the surface temperature of the inlet vs outlet. Someone please post what that needs to be; I've never had this issue before.

 

[east302]

Here is the diagnostic from the 98 manual. Do you have any exhaust leaks on the passenger exhaust?

If you have a scanner, graph the up and downstream O2 sensor voltages together. The upstream will oscillate (lean to rich) while the downstream should be relatively stable. If it mimics the upstream, that means the catalyst isn’t doing well.

You could try swapping the downstream sensor with the other bank to see if the code follows.