This info may or may not be any use but on a Vauxhall I had it did this. The car wouldn't start and the engine warning lights came on so I borrowed a snap on code reader and it just said it couldn't connect. After alot of fucking about I found the reason it wouldn't start and the code reader couldn't connect was because the earth had corroded away on the engine ecu. New bit of wire to battery problem solved and the tester worked as well after this.

Might be worth you running a temporary earth lead just to rule this out, as it will only take you 5 mins.