I know people probably have asked this already but I’m a little slow and dumb but how would I check my cel with a rywire harness and my car being obd 1 with a b20b p75. But the thing is the cel only comes on when the cars on so how would I check it with the car on ? Also I have a 90 ef base hatch

 

If you haven't figured it out by now- jump the service check connector with a paper clip or wire, it's the 2 wire plug by the footwell side under the glove box, turn the key to on but not cranking position and watch the cel light blink to display your codes.