ECT and oil pressure gauges acting funny on key off 2000 silverado 4.8 sc

Hi. I started my 2000 silverado 4.8 sc yesterday and this is what happened. I got code p0118 ECT sensor circuit high and p0122 TPS sensor circuit low. I cleared both codes and they did not come back and all was running fine . Could a low fluctuating idle at start up cause these codes . My idle has to be worked on in the tune at startup but i have had this idle for a couple years now and this has never codded anything. I noticed that at key off my ECT gauge crawls back up tp about half and my oil pressure gauge crawls also to about quarter but as soon as i move the key to ignition on they both zero out as normal as the engine is not warm at all and when i start the truck the oil press gauge is normal and goes up a bit if i gas it and the ECT rises to normal then when i shut off the truck with the key out both gauges crawl back up and they should be at zero . I noticed when i plugged back in my scan tool to see if i had any codes my ECT gauge would zero out but not the oil press, this is when the truck had cooled down for a few hours. Could both gauges be causing this or one because they could be on the same circuit as i still have to pull out the schematics. I am sure someone has seen this before me . This all happened yesterday as i never had this before and sense the last time i used my truck nothing has been touched .I could go down to my truck in the garage and gauges will be showing half ECT and Quarter oil press no key on and i even tried removing the battery and the same reading . Thank you. Eric.

Oh wanted to add that i believe the oil press sensor sends its signal directly to the instrument cluster Gauge and not the pcm . I have another used press sensor sitting around , it is a bugger to get back at the top of block as the firewall is very close and i do have a mp112 that is very high but have to get it done . So maybe it is the oil press sensor but it seems to be good . I know for sure the ECT sensor does feed the pcm , if one was to cause the other to bug i think it would be the oil press sensor because when i plug the scan tool in the obd2 port the ECT gauge is ok . Not sure though. Thank You . Eric

You have ground and/or other wiring problems.

Thanks for getting back . That is what i am thinking also. I find it surprising as my truck has just been parked in my heated garage and apart from starting it , it hasn't moved but a couple of feet sense the two months the last time i started it. it is far from rusted and has but 60K on it only in summer but it can be something like you mention as i noticed when i open the door the coolant temp goes to off and then back up like there is 12v with very low amperage getting through with key off and when the door in open the dome light drains it but the oil press gauge stays at about quarter and is not affected . The gauges work fine when key is on . Nothing has been touched on the truck except for having a small charger to maintain the battery but i have had that on for 4 years now. I will have to work on this. Eric

You will have to inspect the wire harness and connector terminal pins, particularly related to the items you mentioned above and ralated to grounds.

Maybe some connector got wet or some terminal pin bent or some wire fractured.

In my case, rats ate the "environment friendly" wire insulation in the DBW subharness.

Thanks for getting back . It cant be the gauges themselves as they work fine. I am not an expert at electronic circuits but i am wondering if it could be a cold solder joint in the cluster . I am going to take a look at it when i get some time sooner than later. I used to repair audio products but mostly car stuff years ago alongside another older tech and cold solder joints were very common problems because of shock from driving and vibration . Also i will try pulling some fuses to see if an identifiable circuit is doing it . There is voltage getting through so like you said it could be a bad ground or bad wire , the truck has low mileage and is clean but i bought it new is 2000 , wow almost 25 years . Thank You. Eric