Arrived at the customers home because of the heater not working properly. Was greeted at the front door and was shown where the thermostat and furnace were located. Customer said heater had been working ok until it had stopped. Looking at the furnace I noticed it to be locked out on a high limit fault and started looking at reasons that the high limit could be tripping. Found filter to be completely restricted so I removed the filter and showed customer. Reset the system to clear faults and restart the heating cycle. System fired as normal and ran for 15 min with no problems. Customer said that she would go to the store and get filter for the system. No further repairs needed at this time.