95 2.7L p0125 need help fast

So my sons truck is throwing a p0125 code on a 95 2.7L, this is after he replaced the entire exhaust system with walker exhaust from rockauto. Manifold, down pipe, cat, muffler, and tail. He also replaced both o2 sensors as well. After he fired it up and test drove, it came with the p0125. From reading about that code, it is the B1S1 that is failing. After checking voltage and resistance and ground, we are at wits end to what the issue is. Any input on what to look for or what to do would be greatly appreciated as he needs to run it through inspection by the end of the month. Thanks again.

 

And a pic just to make it fun

 

P0125 is insufficient coolant temp for closed loop operation. This typically means that the engine is not getting to operating temp quickly enough. Not always, but usually this is due to a bad thermostat that is stuck open.

 

Everything I have read says the b1s1 o2 sensor is the culprit for the p0125 code, but, thermostats are cheap, and will give it a shot