Replace both O2 sensors ?

No problem, but I pretty much just summarized what @treyus30 was saying lol. Its hard to diagnose much over the web without knowing the exact code you're dealing with. If you can get that from your mechanic and post then it would make it much easier to know what to advise you with.

 

hmm P0138...well since a new downstream sensor didn't fix it then I suppose there are some other options but they require some diagnosis. Most of what I'm reading about P0138 on Toyotas is that a new sensor usually fixes it, and if it doesn't then looking for damage or a short in the wiring to the sensor is the next step.

some info on it

https://www.toyotanation.com/threads/o2-sensor-po-138.403957/

https://www.700r4transmissionhq.com/p0138-toyota-tacoma/

Maybe I missed it in an early post but what was your mechanic's advice on the next step?

 

Ah ok, yeah these trucks do not mix well with aftermarket sensors so the Denso should do the trick.