On a 1995 GMC sierra P/U 5.0L engine. The

Check the exhaust for leaks by briefly holding your hand or similar over the tailpipe when the engine is idling - if there is a leak get it fixed, if there isn't consider extending the tailpipe length.

It is also worth checking the door and tailgate seals and adjustment...

It probably smokes all the time , when moving you don't notice it > What color smoke ? White - coolant is getting in to the combustion camber , head gasket , intake manifold . Blue - oil is being burnt - rings , valve seals , Black - running rich , leaking fuel injectors . leaking fuel pressure regulator . Smokes from were , exhaust ? Engine bay etc....

P0420 - Catalyst System Efficiency Below Threshold (it not cleaning the exhaust up)
Possible causes- Three way catalyst converter Bank 1
- Exhaust tube
- Intake air leaks
- Fuel injectors may be faulty
- Fuel injector leaks
- Spark plugs may be faulty
- Improper ignition timing
- Engine Control Module (ECM)

look for a serious exhaust or intake leak. look for bad wires to the o2 sensor or map sensor. just for starters.

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u most likely have a worn clutch or air in the system from a hydraulic leak this has no adjustment .

you have to remove the drive shaft, the cover around the bottom of the trans. Remove the torque convertor bolts,remove the bellhousing bolts from the trans. You may have to unbolt the exhaust bolts. Slide the tans back remove the flywheel to access the oil seal.

could have a cracked head and blown head gasket blowing water out the exhaust

My 2005 GMC 2500HD (8.1 lt) had the same problem. I just had an exhaust leak fixed today. The right front manifold bolts were broken and the manifold was warped. The repair was 6.5 hours labour plus gaskets, seals, bolts and the manifold machining (parts were $130 CDN).

Just a thought, have you checked yout TPS(throttle position sensor)? I had similar problems with my 89 GMC Jimmy 4.3l, however a code 44 is strange. Also check to see if your problems go away if you advance the timing a few degree's.

Hope this helps