My car started and was running fine. When I got back in it at store. Put key in ignition the dash gauges went haywire and car wouldn't start. Thought maybe battery? Got it jumped although the idle was fluctuating I drove it home. After parking it I tried to start it again. Same thing gauges haywire wont start. Checked battery good full, alternator good. Replaced fuel relay cause when u remove that relay gauges preform normally. Soon as put in new one gauges went haywire again. I don't know what else to check. I have no idea what is wrong with car?
Use an inexpensive digital voltmeter to measure the battery voltage. It must be 12.6 volts. If you find closer to 12.2 volts, it is good but discharged. If it's around 11 volts, it has a shorted cell and must be replaced.
Charge the battery if necessary, then once the engine is running, measure the battery voltage again. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator. Simply saying something is good won't cut it. I want to see numbers.
Also look in the under-hood fuse box for a large bolted-in fuse and for a smaller red positive battery cable bolted to it. Be sure all of those are clean and tight.