I ran my AutoCal the other day to look up my error codes and the AC came up with 3 of them.
P0135, Upstream O2 Sensor (left cylinder bank) heater performance
P0327 Knock Sensor signal freq outside normal range
P0332 Knock Sensor signal freq outside normal range

According to the Haynes manual, the 02 sensor has a feature were it allows the sensor to heat up so it can do its thing. I think that's what wrong with mine. Being that it's the heater performance I dont belive its affecting the air fuel mixture, but I might be wrong about that. Either way I'm planning on replacing them both just to be safe. Anything I should know about doing this?

My concern is the other 2 codes. I thing my Tahoe has 2 knock sensors. I don't think the engine is knocking or doing anything weird, at least I can't hear anything obvious, so I think the actual sensors are bad.

So my question is, how difficult is getting to the sensors to replace them. The Haynes manual has me removing the intake manifold, and to be honest I'm a bit intimidated by that. Are there any considerations In should know about if I decide to do this?
Is there a gasket that needs to be replaced when removing the manifold?

Thanks
R