(So Cal Plumbing Heating Air Conditioning technician). Went to a service call today, furnace would not light. This was a 4 ton carrier unit installed in a garage, it had Asbestos wrapped ducting and was in poor condition for a 1998 unit. This was a basic 80% upflow unit with the return cut into the side, we tested for gas leaks as we always do before any diagnosing and we found no leaks so we we able to proceed. We did a complete visual inspection inside and out and besides being dirty, rusty and covered in Asbestos everything seemed to be in order, no missing parts or signs of a previous hack job. We then moved onto the T stat, tested voltage on the batteries and were OK, ran a continuity tests on all terminals and found R and W were shorted so the unit was stuck on calling for heat. We replaced the T stat and then tested the communication wires after disconnecting from the control board. Wires were fine with no shorts. We then proceeded to jump the R W out at the control board and the unit still wouldn’t light, Inducer spun and gas valved opened but no hot surface igniter, we then tested voltage from the control board to igniter and found our problem, bad hot surface igniter. Replaced the igniter with a brand new one that we stocked in the truck, jumped it out one more time and it light right up. Reconnected communication wires and T stat, re installed door switch and doors, programmed new T stat, set to to heat and we had one happy customer. We recommended a full service and that they have their Asbestos abated as we always to and gave them a link where they can buy their replacement filters online for cheap. We were then off to the next call and future satisfied customer.