Failure in Lennox 80 MGF Furnace EGC Control Board

I offer this bit of info for googler's trying to fix their furnace...

A few months ago my furnace started acting pretty odd. At temperatures below about 40 degrees, the gas valve would open almost immediately after the thermostat would call for heat. The ignition sequence would fail. (Not sure exactly why). Normally a 45 second pre-purge by the combustion blower should occur before the gas valve is opened. Next the valve is opened and the spark unit is activated for a few seconds to light the gas.

At temperatures below about 60 degrees, the air blower would not run after gas is shutoff. The blower is supposed to run for a couple of minutes after flame is off to purge the exchanger of leftover heat.

The furnace is located in the attic, a pretty bad environment here in texas. Attic temps get pretty high in the summer, and below freezing in the winter.

My first experiment was to take a blow dryer on a cold morning when the furnace would not light and locally heat the EGC control board. Sure enough, even after a short heat, the furnace would fire. My next step was to try to determine what was wrong with the board. After a little research, I discovered the electrolytic caps used on the board were the most likely culprit. I was surprised to discover electrolytics have MTTF of about a thousand hours at temp/voltage. Their lifetimes improve when not operated at full voltage/temp, but still not very good compared to other electronic components.

I pulled all the electrolytics and tested them. Only one (C2) was a little low. Next I put the C2 cap in the freezer with the cap meter leads connected. Within a few seconds, the cap value started to drop. Eventually it went from the 40uF initial value (spec was 47uF) to less than 10uF.

I decided since I'd already unsoldered all the caps off the board, I'd go ahead and replace them all with the best caps I could get on short notice. I went with a Panasonic FC model cap. I also noticed the larger form factor caps had better lifetimes on the spec. I ended up using 63V models for all the 47uF caps and 100V models for all the 22uF caps in hopes the higher voltage/bigger case will result in a longer lifetime. Lennox did use 105C caps, but I could not determine if they were extended life models because the cap markings did not indicate a specific cap model.

After putting the new caps in & re-installing the board, the furnace operates as before. Total repair cost was about 25 bucks including the FedEx delivery charge of the parts from DigiKey. Please note, if you don't know how to solder a PC board, don't attempt this. De-soldering is not difficult, but does take some practice.

I'm tempted to replace the caps in the second furnace in the house as a precaution.

Upate 10/05: The unit that had failed ended up getting replaced this summer because the A/C unit started leaking. I figured I'd take advantage of some of the A/C rebates available and upgraded the entire system, as the high efficiency A/C unit required a variable speed blower in the furnace. I note the new lennox G60 series put the circuit board inside the air flow. I'm guessing this minimizes some of the temp extremes since the inside air blows over the ciruit board. Good idea on lennox's part I think.