Troubleshooting a Rheem furnace that will not light consists of checking the spill switch, the HSI heater and the ignition control. Checking these items requires an ohmmeter, a voltmeter and a screwdriver. Completion of the project takes approximately a day.
Check the spill switch
In the case of the Rheem furnace, hot air is released through a vent on the top of the appliance. However, if this vent is blocked, it can cause a backup of dangerous gases, specifically carbon dioxide, within the vent hood of the furnace. Inside the hood is a sensor called a spill switch. If the spill switch has detected the build up of hot gas in the hood, it will open, allowing electricity to run through the switch and turn off the furnace. This switch needs to be manually reset in order for the furnace to operate.
Check the HSI heater
The HSI heater is the component of the Rheem furnace that ignites the gas. It's also commonly referred to as a "hot stick." This hot stick is very fragile and has a tendency to break if used frequently. Checking the HSI heater is relatively easy and simply requires taking the two probes of the ohmmeter and holding them against the hot stick. If they register that electricity is flowing through the component, then the HSI heater is broken and needs to be replaced.
Check the ignition control
Checking the ignition control requires turning the furnace or thermostat off and on. This may reset the ignition control pane, in which case no further troubleshooting is required. If this does not reset the ignition control panel, then while the power is on to the furnace, touch the probes of the voltmeter within the space designated by a "TH" and "GND" on the ignition control panel. If 24 volts are registered, then the ignition control panel is broken. If no electricity registers, then the problem with the furnace is not with the control panel.