Customer present free diagnostic service call coupon. Issue was resolved in the 1st half an hour. No charge Upon arrival set thermostat to call for heat. Furnace is on/running but does not fire. Check control board which was displaying error message of 4 flashes. 4 flash indicates open limit switch. Remove wire and checked continuity on all limit switch. Found limit that was open. Limit switch is resettable. Reset limit, check all electrical wiring, checked with customer to verify that no vent is being blocked. Blower motor is ok. Customer replaced dirty restricted filter while on site. Explain to customer that some case limit switch can be faulty. Where switch would need to be replaced. After reset of limit switch system was ran and monitored to be ok. System is left operational @ this time. View visual findings from this job here - https://visionshare.xoi.io/?id=XA-90AE-DB63F3761B2F42BD871A8B7C9F4AF4A1 Recommend for customer to follow up with heating maintenance