If your car wont start can you still run a compression test on it?

Compression Testing

Yes, IF your battery can "crank" [turn the engine over] then a compression test can be accomplished even though the engine will not start and run.

You need to remove all of the spark plugs to reduce the "drag" caused by all of the other cylinders not under test.

Attach the compression tester to one cylinder at a time, and then crank the engine for a few seconds. Read and write down the reading, and then move to the next cylinder until you've check them all.

If your battery is low, you may not get full readings, but for comparing cylinder readings, the ratiOS will be about the same.