Oil pressure gauge pegged at120 on 2003 pontiac bonneiville ssei from startup to shutdown was wondering if it was the oil pressure sending unt

test the sensor and wiring connections

have the oil pressure tested if its good it may just be a bad sending unit.

check for a pinched wire that goes to the gauges.. its grounding out.

Sounds like a grounded wire. Most senders change resistance to the ground circuit of the gauge. If it pegs there is no resistance to ground and probably a shorted wire.

The one by the filter is the oil pressure sending unit.

Probably just the oil pressure sender, at least that is the first thing I would replace.

A number of things could impact this, a dirty oil filter, different type or weight, of oil or quite simply wear within the engine or oil pump (or combination of both)
First I'd get a gauge under the hood and get a true pressure check on the oil, perhaps the sending unit is failing. If it reads fine be certain the filter is clean. If still nothing it is within the engine, you can try to use a heavier oil that may increase your oil pressure.

I hope thi may have assisted you,
Tom

Oil pressure 'sending unit' is bad. It needs to be replaced.

The oil pressure sending unit is located behind the oil filter, Threaded into an aluminunm housing. It has wiring that plugs into the sending unit. The oil filter has to be removed and you will need a deep well socket and a 6 inch extension. Change the oil and the sending unit together.

it is probably the oil sending unit if you have access to a direct reading oil pressure gauge put it on and if oil pressure is reading normal replace sender