Furnace Repair: Spark ignition with no pilot : Weatherwise Heating and Air Conditioning
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Furnace Repair: Spark ignition with no pilot

by Weatherwise on 02/04/12

Here is a list of common problems :

Gas Furnace - Spark Ignition / No pilot

Draft motor does not start

Draft Motor 

(Check to see if motor is seized or hot, check load voltage to motor)


(Check 24V at C & W)

No Spark to ignite main gas

Open safety in control circuit 

(pressure switch, high limit, roll-out)

Ignition Module / Control board 

(Some ignitors are in the control board, 24V signal to the ignition module through the safeties, but no spark)


(No call for heat)

Ignition wire / Ignitor 

(Loose connection or shorted wire)

Draft motor 

(safety monitored by “pressure switch” in control circuit)

Ignitor sparks, but no ignition

Ignition Module / Control board 

(Some ignitors are in the control board / No 24 V. signal from the ignition module to the solenoid on the gas valve)

Gas valve 

(24V signal at the gas valve, but no or low gas through the solenoid on the gas valve)

Improper ignitor grounding 

(often ignitor sparks, but fails before gas is activated)

Burners ignite, but shut off after a few seconds

Flame sensor

(Remove and clean flame sensor with sand paper)

Pressure switch 

(Check pressure switch tubing for obstruction)

Ignition to burners, no blower motor (furnace will overheat and lock out /high limit)

Blower motor 

(Check to see if motor is seized, check load voltage to motor)

Control board 

(internal relay not sending high voltage to the blower motor)

Run Capacitor 

 Ignition to burners & blower motor runs, but furnace overheats and locks out high limit

Dirty air filter

Dirty evaporator coil

High gas pressure

(Check outgoing pressure from the gas valve, adjust if needed)

Restricted ducting

(Test furnace with air filter removed & check temperature rise across the heat exchanger)

Blower motor runs continuously with no burner ignition


(Check 24V signal at C & G)

System lock - out

(Check diagnostic codes at the control board)

Flames roll out of heat exchanger

Dirty burners 

(Remove and clean)

Blocked venting 

(Inspect flue piping and remove restriction - make sure there is more vertical rise than horizontal and the horizontal rises ant 1/4” per foot)

cracked heat exchanger 

(Perform visual inspection, video inspection, carbon monoxide test, or trace gas test as needed)

Troubleshooting a gas furnace for repair is a step by step logical sequence.  It is always important to look for the obvious.  For example, it is important to know what is working to quickly eliminate it from troubleshooting.  In the Denver area heating is crucial, so a technician will encounter a wide variety of situations of types of heaters from a standard gas furnace, high efficiency gas furnace, or a hot water boiler. Routine furnace cleaning can eliminate a lot of intermittent failures. Weatherwise is a Denver metro based HVAC contractor, with the tools and experience to handle all of your heating and air conditioning service needs.  Feel free to call our company for any questions you may have.  

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