Gas furnace ignitors

When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas.  If you do not have the ability to do these operations safely, do not attempt them.

When the modern furnace does not light, one of the simplest troubleshoot methods is to check the ignitor.  
The HSI or hot surface ignitor is a common method of igniting the burners.  

The silicon carbide ignitor shown on the left is common.  
It reaches over 2000F to light the burners.  
This ignitor is quite delicate and must be handled carefully.  


On the left is a failed HSI.  The white spot is the crack.  

HSI mounted near burners.

This is an example of the mounting of the silicon carbide ignitor.

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Below is a video showing how it should look when energized.

The cracked HSI above is obviously failed.  It will not glow at all.  If tested with an ohmmeter, it will read open.  However, the failure is not always obvious.  There are readings on the ohmmeter that will indicate a failing HSI.  The video below gives readings to look for.   

Silicon nitride ignitor

This ignitor is used in the same manner as the silicon carbide ignitor.  It is much tougher than the silicon carbide unit.  It is not a direct replacement for a silicon carbide ignitor.  To test this ignitor, the ohmmeter should read 13 ohms.

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The silicon nitride ignitor

Here is the mounting of the silicon nitride type of ignitor.

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If the HSI works normally but no flame comes on at all

When all sequencing is normal, but the burners do not light, Check here

Other codes

There are other codes such as reversed polarity which means the power for the furnace has been hooked up backwards.  This would only happen upon new install or after changes have been to the wiring. 
There is a shorted gas valve code code on some furnaces that that indicates the gas valve has failed and must be replaced.