I have a 20+ year old Kenmore Natural Gas Furnace (intermittent ignition) (model 867.769030).

It just started a rapid, constant clicking and will not start up, and makes no other noises.

The clicking is coming from sparking in the combustion air relay.

Any ideas of what the problem may be?

Thanks in advance.

Here is the Sears part site. Just put you model number in. There will be a diagram or part number list.

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It could be a flame sensor or a plugged up pilot orifice. Go and check the pilot orifice first.

I noticed that the exhaust/combustion air blower does not start blowing. Should that kick in before the gas starts. If it should, is there anyway to trouble shoot it without replacing it.

Check to see if you are getting power to inducer motor if you get 120v and motor not coming on motor is bad and needs to be replaced... that is first in sequence of operation

When I depress the interlock switch that the blower compartment door usually engages, while checking for power at the plug that the wires from the motor plug into, I get power that cycles on and off very quickly - at the same speed the sparking is happening in the combustion air relay in the control panel. Is this normal, or maybe the relay is bad?

It does sound like bad relay,should never see sparking on any electrical control

Check or replace the relay. I was thinking you said the door swirch was not working.

The door switch is good because I get steady power on the upstream side when the switch is engaged. I bypassed the relay and got steady power to the blower motor. It didn't start. Still not sure if the rapid, constant clicking in the relay is a sign of a bad relay or just caused by bad blower fan. However, now it sporadically does nothing when I engage the door switch, and I have to keep the button on it's face pushed in to allow power to flow through it. I'll assume either one, or both, need to be replaced, and post how it turns out.

Clicking of the relay could also be from low voltage. Check to make sure you have 24 volts for your control circuit.