Northern has 10 class 158/9s that based on the time between overhaul (8 years) need to go through a C6 overhaul line with Northern's & Eversholt specification document "EV/VI1586 Issue 3". The specification for the C6 focus' on; AIR SYSTEM (Excluding brake equipment)/ BRAKES - AIR/VACUUM/ BODYWORK (Excluding interior trim)/ BATTERY/CONTROL SYSTEMS/ FLOORING/ HEATING/AIR CONDITIONING/ INTERIOR TRIM /FITTINGS/ FUEL ...