depending on the error, you could have used a parts store scanner to scan it, but i think in CA it is not allowed for the stores to do it for you... if you meant, like on the old E32, E34, E36 models, when you could read the code by flashing the CEL - no, you can't do it with the E38, E39, E46, E53 cars, you need to connect to a 20-pin port under the hood on older versions or strictly 16-pin OBDII port in the cabin, and need either a code reader/scanner or a laptop with specific software...

horizontal line on the display - this is very vague... are those physical scratches? are those dead pixels? are those radio interference lines/out of sync? more details, if you can, please...