Search on here to learn how to pull the engine trouble codes using a paper clip.
Yrs ago one of mine alarmed as soon as it started due to thinking it didn't have oil pressure. I got the oil pressure test port and screwed it into the block, hooked up an old school oil pressure gauge with clear tube and verified I do in fact have good oil pressure so I determined the oil pressure sw (buried deep under the intake manifold) is bad. I routed the clear tube and pressure gauge to my control cables just outside the engine and mounted it there. I then wired in a "oil pressure" on/off dash switch that is spring loaded to the oil pressure "on" position. To start that engine I have to fool the ecm.
Start procedure: Push sw to no oil pressure position, turn key on, crank engine, after engine starts release sw to oil pressure on position. Look back at oil pressure gauge, go about my day.