Q: code p0591 has check engine light on, but car has no crusie control, what else is it on for?

According to my records, the code you are receiving is for a powertrain issue relating to cylinder 4 being out of balance. On diesel engines this can indicate a glow plug issue. This can be related to the fuel composition sensor circuit. You may want to look into the sensor, or the fuel system. You should also look into cylinder 4 and make sure the ignition system is working well. This means inspecting the spark plug and wire. Are there any other issues going on with your car at this point in time? If you are having the vehicle scanned with a generic reader, the codes may not come up as accurately as they can with an upgraded version. Manufacturer specific or equivalent scan tools may want to be run on the vehicle for a better analysis.