BASIC GAS FURNACE FIRING SEQUENCE (newer style furnaces with control boards)
  1. Call For Heat- your thermostat sends 24v back through the "W" (usually white wire) to the furnace control board which then starts the heat cycle.
  2. 120V is sent to the inducer/vent motor
  3. a 24V pressure switch then closes *if the inducer/vent motor started correctly and has enough volume of air displacement to create enough vacuum to activate the switch. If you get get a PRESSURE SWITCH failure code at this point it does not necessarily mean that the switch is bad. Check for obstructions in the Flue (birds nests, Crushed flue cap (recent roof repairs are common causes of this) squirrel or rodents etc also are commonly found. Also check the rubber tubing that runs from the pressure switch to the heat exchanger or vent motor as these can crack or split which will also cause the error. The last thing to check is the port that the tube attaches to as it creates a slight vacuum sometimes rust can block this tiny port so it can be as simple as using a paper clip to clean it out. 
  4. Once the pressure switch "PASSES" voltage is then sent to the IGNITER (HSI) Most igniters are 120V however there are some now that are 80V and some even 24V so be aware also that the outside of these igniters even if they do not glow carry voltage so if touched they will shock you. If your igniter fails to glow at this point the furnace will usually code out - "Failed to Ignite" and your igniter may be bad if voltage is going to the igniter, if not you may have other issues or possible control board failure . Look for small cracks or broken igniter or check for continuity and/or OHMS. 
  5. If the HSI (Igniter) glows power will then be sent to the gas valve (usually 24v) which will then release gas to the burners. TIP: At the start of the season some gas valves can be stuck and a simple tap on the side of them with the back of a screwdriver or other tool can un-stick them. 
  6.  If gas ignites *ALL OF THE BURNERS* a flame sensor then will sense that flame is present and continue its cycle. A common cause of failure is the flame sensor. It can be cleaned with emery cloth or light sand paper and in most cases will then work as it should again. *SEE - "FLAME SENSOR ISSUES" in the Knowledge base for further diagnosis and repair. 
  7. Once the flame sensor senses the flame correctly a message is sent back to the furnace so the blower motor will then activate.
  8. If the blower motor fails to start or a blockage exists in the airflow the "HIGH LIMIT" may trip in which you then get this error in which case doesnt always mean the Limit is bad and generally even if it is bad - there is an underlying fault causing this to trip and/or go bad so look for badly blocked filters, blocked ac evaporator coils, crushed ducting and even too many vents closed off will also cause it. You should get around 35 degrees of temperature split between the air going into your furnace and the air coming out. If you have a higher temperature split this will generally mean you have an airflow restriction which will cause the high limit to trip