How to Reset Most Car Computers Without Specialized Tools

Recently, a user posted a problem in our Ask a Mechanic Forum about his Cadillac Deville putting up multiple warning lights without apparent justification. Obviously, anytime a warning light comes on, it should be thoroughly checked out. Sometimes, though, it could be due to another problem.

For example, while working on a Honda Civic not long ago, I opened the engine's fusebox (computer module) to swap out a relay. I had the wrong relay, so I left everything open (the hood had been removed as other repairs were also ongoing) and went to the parts store to swap the relay out. While I was gone, it rained for a short period, but heavily. Upon return, I replaced the relay and closed up the box.

When I started the car, nearly every warning light on the dash was on. I pulled data from the computer and a very long list of error codes was dumped. Of course, since I have the reader, I was able to clear them all, let the car sit for a while, the restart to find no new codes (after waiting, of course).

Had I not had the right interface to pull codes or if I'd had a reader-only (units that cannot "send" to the car, common with cheaper systems), resetting the codes would have had to have been done manually.

Resetting the computer manually

In that case, the process is simple. To clear the computer, you must remove all power from the system so that the codes are "forgotten". You do this by simply disconnecting current power and then grounding out whatever power is residual.

Remove both battery cables (positive and negative) and then connect them together. You can do this with electrical tape, zip ties, or clamps. Keep the cables away from the battery terminals (using a bungee, you can "hang" them from the open hood as an option) and let them remain connected for at least half an hour.

Clean all terminals and connectors thoroughly (water and baking soda work well). This should take five or ten minutes if done thoroughly. Then reconnect everything (about 40 minutes have passed at this point) and start the car again.

No codes should be present as the system has been drained completely. Be aware that you will likely lose presets and other non-permanent data in your stereo and accessories.

If codes continue throwing, you may have a more serious problem.