O2 heater relay?

[Daskoff]

I originally had a check engine light related to the downstream O2 sensors, I changed all four sensors with Denso units from Rockauto. I got the errors again - specifically P0136 and PO156 indicating the downstream O2 Sensor Circuit Malfunction. My mechanic thinks it may be the 02 heater relay which may be the problem but I would imagine that there should be a code for the relay if that was the problem - based on some postings I've seen while searching for this. I am really stumped, did I receive two bad O2 sensors? Doesn't seem likely. His checks suggest that maybe the O2 sensors are not heating fast enough or the ECM can not "see" the O2s heating - therefore it may be the relays. Not even sure where the relay is or how much it costs to get a new one- if it is the problem. Anyone have any ideas? This is the last problem I am having and this truck has been in service (fixing all the deferred maintenence) for longer than it has been at my house.

Thanks

 

[ihadmail]

I'll look at the electrical portion of the FSM and see if there is an o2 relay.

 

[ihadmail]

I am sorry but I got busy at work and couldn't get around to looking at the FSM for you. I don't have it saved on my laptop at home.