ICP Furnace Showing 80% Limit Fault

Most gas furnaces have several safety limits, main limits, roll-outs, 80%’s will have a DSS (Daft Safety Switch) These safety switches are look at by the CPU on the main control board all the times. 

An ignition fault is a common fault with a furnace from time to time. It could be caused by several things. An ICP 80% furnace will try to light a total of four times before the furnace locks itself out on a Code 6.

This fault can happen during the beginning of a trial heating call or after the furnace lights if it loses flame sensing. If a furnace loses a proven flame before the blower cycles on, the blower will run for the selected off-delay time set on the control board.

These are the things that need to be checked if you have this fault. 

First: check that the green/yellow wire on the gas valve is connected to the sheet metal cabinet to ensure there is a good ground to the control board and gas valve. Make sure the gas shutoff is turned completely on, and also verify the gas valve is turned on.

Next: check the gas inlet and manifold pressure. A furnace with low pressure will have problems lighting from time to time due to under-firing and can light if other gas appliances are not on at the same time. Low inlet pressure can also be caused by the gas line not being large enough to supply the furnace with gas. Check your line size to ensure you have a properly sized line.

After that, the flame sensor is also an item that will need to be checked. Specifically, the microamps. A normal reading will be between 4.0 and 6.0 microamps. If there is oxide buildup on the flame sensor, clean it with fine steel wool.

Lastly, check the crossovers to ensure adequate flame crossover on the burners. This will cause the flame not to reach the flame sensor.

For more information check the installation manual that is supplied with the product as some models will also have a troubleshooting manual.

 Sources, Links & Media:

• HVAC Partner Installation Manual.