P0138 GMC -

Question

Replaced the rear O2 sensor in 1996 GMC 454 V8 due to Code P0420. Code P0138 came on after about 100 miles of driving pulling a trailer.

Is it electrical, a bad sensor or something else?

Answer 1

P0420 Catalytic low efficiency

P0138 H02S circuit high voltage bank 1 sensor 2

This is the catalytic monitor H20S sensor at bank 1 . Circuit high mean the ECM did not see a voltage use,or the 02 has  a open circuit  The way your vehicle works the rear 02 sensor becomes a open circuit when the catalytic is below 600 degree F.In other wodrs the catalytic is no lit up. More than likely you have a bad sensor when you had P0420 the H20S was reporting the condition , After the replacement  circuit high