Tricky ,any warning lights on (i’ll no doubt get this wrong lol) on euro 6 is a fail but not euro5 and before …it was part of the change in may …We phoned the station before a test for a vehicle with a warning on and they said it isn’t a fail//Then they wanted to fail it when presented …Luckily our lad reminded them of the call and it was passed lol…Obviously it was repaired before being returned