93 suburban problem

have 2 problems 1st the check engine light comes on when pullimg my trailer and then goes out the code is 44 and is the o2 sensor lean . How do I check this to make sure it is the o2 sensor. Next question is the tranny when pulling the trailer doesn't seem to down shift or is really spongt feeling when going up hills the truck is a 3/4 ton with a 454 and it seems like it has no power I have had the intake manifold replaced the plugs wires cap rotor and egr any help would be great thanks Rick Any one have any answers or know which tread I should post in.

I have an 89 my dad has 95 gmc - both 350s - not 454s, but...

- Check your base timing first - unplug the tan/black (breakout near brake booster or under heater - dunno for sure - check label on rad) (note you will get a code 42 and that's OK- I think you set for 0 degrees - again on label)

- I assume you don't get a lean any other time? Possible problem is fuel pressure is low - check plugged filter and pressure regulator fuel pressure.

- Also check your big vacuum line off the intake to the tranny - might have a break - disconnect.

Let me know if anything helps....