[rennat_2006]

P0135 is a 02 heater circuit, could be heater element in the sensor or a ground issue. I cant remember what years silverados and other fullsize trucks/suvs it was but they had a common problem with a bad ground causing that code. The 02 sensor body grounded through the exhaust pipe itself, quick test was to command the heater on with a tech2 and if it didnt work take a piece of wire strip an inch or so bare and wrap it around the sensor body then put the other end on a good clean ground and try it again.

Engine doesnt run off of the o2 sensors until it hits a set temperature, cold starts are all off of the "base" tune aka open loop.

Easy check for intake gaskets leaking is grab a can of carb clean, brake clean, etc and spray down both sides of the intake where it meets the cylinder head BEFORE you start it the first time for a cold start. Spray it down real good then run around and fire it up real quick, if it idles higher than normal that confirms it. You can also smell the brake clean/carb clean in the exhaust if you put enough down.