P0138 Code after recent alternator replacement

About two months ago I replaced a dead alternator. Last week I started hearing a ticking noise coming from under the hood. Since then, I was out of town a lot so didn't drive much, but today it gave me the check engine light and a p0138 code.

The ticking accelerates when i hit the gas, but goes away as the engine warms up. The serpentine belt looks intact, and was replaced when I changed out the alternator. p0138 is the oxygen sensor, but I'm not really getting any other symptoms that seem like it would be that (yet), and I don't think it's a coincidence with the alternator.

Thoughts or suggestions?

 

Exhaust leak will cause this. Is the truck fairly rusted?

 

The ticking accelerates when i hit the gas, but goes away as the engine warms up.

This sounds like an exhaust leak.

 

x3 on the exhaust leak. Since you are throwing the code, it's likely a leak before the downstream 02 sensor, likely an exhaust manifold leak.

 

No ticking this morning and the check engine light went away. How would I confirm an exhaust leak?

As for rust, I wouldn't have thought so, but this is the start of the exhaust:



Is that the o2 sensor?

I don't seem to have any of the other symptoms of a leak - no smell, no rough running, no rumbling sound, no gas pedal vibration.

 

That's one of your O2 sensors. If you currently have no codes or noise, I would leave it alone. Some people run there hand to feel for air leaks but I would do a much safer method if you have a shop vac or leaf blower. You can do this with engine off and even spray soapy water around the manifold and pipe connections to locate the leak. It's likely one of the gaskets failed over time and needs replacing. Shouldn't have any drivability issues besides the annoying tapping noise and CEL code.

https://www.youtube.com/watch?v=c6QE4dSJrt4