The P0135 code is specific to the Oxygen Sensor Heater Circuit Malfuntion (Bank 1 / Sensor 1 ).
The first thing you need to know is whether you have a 50-state emissions setup or 49-state emissions setup. You can determine this by crawling under your rig and having a look at the exhaust. If you have a single catalytic converter parked right next to your rear driveshaft with an O2 sensor behind it, then you have the 49-state setup. If you have two mini-cats up on the driver's side of your engine then you have the 50-state setup which means you also have four O2 sensors instead of only two.
If you have the single-cat setup, you will have one O2 sensor before the cat and one after. The P0135 would refer to the sensor BEFORE the cat.
If you have the 50-state setup, then this code will refer to the O2 sensor BEFORE the mini cat closest to the FRONT of the engine block.
Also, try a different mechanic. You don't need to scan to know these things....