All gauges working except tachometer on 2003 Chevy Avalanche

A ground is most likely the case

Common problem on the 03-06 Avalanche instrument clusters, it's caused by failing stepper motors on the cluster's circuit board and it can be repaired. Check this page for more on how to get it fixed - Avalanche oil gauge fix

Sounds like the oil pressure sensor. Thatt is the easiest thing to change. If not its the gauge. Put voltage to the wires from the switch and see if the gauge stops jumping and stays pegged, if it does its the switch Good luck

Oil pressure gauge is not getting good ground. Check wire.

What car is this on. There is NOT a BCM in a 2001 chevy s10 blazer.

Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Silverado instrument cluster repair
Pretty common problem with these. Here's a guide to learn how to fix GM instrument clusters yourself, I used it to fix mine and it worked great.How to guide for Tahoe instrument cluster repair

the sensor is located at the back of the top of the engine, behind the distributor. Could be the sensor or the gauge itself. Probably the sensor stuck.

you should check all the fuses and check the colent sencer on the block and check all the electrical conections coming from the tank if cant find any thing check the cluster and if bad replace

Check first for blown fuses.

Its the cluster assy. I believe there is a special policy from chevrolet for this repair at no cost to you.