Fireplaces, Heating Stoves, Flues and Chimneys - Lennox Furnace pressure switch

I have a Lennox G51MP furnace and have problem that is driving me crazy. The unit has redundant 2-tube pressure switches. If either of the switches doesn't make, the system will allow ignition and flame, but then flame failure occurs within 10 seconds or so. The combustion fan continues to run, and after a timeout cycle will attempt to restart with duplicate results. Troubleshooting lights don't indicate a problem.
I checked the pressure on the switch and got a 1.5 reading on the right side of the plenum collector box and 1.3 on the left. The switches are rated to make at .9. There is no residual water in the collector box or tubing. The right side switch makes, the left doesn't. I have replaced the left side switch with the same result.
Oddly enough, when I swap switches, I get the same issue with the right side making and the left side not.
Trouble shooting lights appear normal with both flashing fast which means the system is calling for heat. Everything works fine if I jump out the left side switch.
I am a retired mechanical engineer and this has me stymied. I am leaning towards a baffle issue in the plenum collector box.
Any help would be appreciated.

The G51MP will not fire if the pressure switches are not closed....
Sounds to me that You have another problem.. odd that you get no led codes.... Sounds almost like the system is not proving flame. That happens about 4 seconds into firing.. it would take 5 failures in a row for this to trip a led code....

test the theory of the pressure switch.. right after it fires.. unplug one.. do you get a code?

Sequence of Operation
1 − When there is a call for heat, W1 of the thermostat energizes
W of the furnace control with 24VAC.
2 − S10 primary limit switch, S47 rollout switch and S21
secondary limit are closed. Call for heat can continue.
3 − Control (A92) energizes combustion air inducer B6.
Combustion air inducer runs until S18 combustion air
prove switch closes (switch must close within 2−1/2
minutes or control goes into 5 minute prove switch
delay). Once S18 closes, a 15−second pre−purge follows.
4 − Control (A92) energizes ignitor. A 20−second warm−up
period begins.
5 − Gas valve opens for a 4−second trial for ignition
6 − Flame is sensed, gas valve remains open for the heat
call.
7 − After 45−second delay, control (A92) energizes indoor
blower B3.
8 − When heat demand is satisfied, W1 of the indoor thermostat
de−energizes W of the control which de−energizes
the gas valve. Combustion air inducer B6 continues
a 5−second post−purge period, and indoor blower
B3 completes a selected OFF time delay.


try this.. follow the chart on page 44 this will give you a step by step sequence of operation in a flow chart.

http://www.maro.net/g51mp/0307i.pdf <------ You now have every thing you need to know about your unit.
Work for an Authorized Lennox Dealer:D

hope that helps a mechanical engineer:rolleyes:
used to live in Huntsville Alabama.. the place is full of them :)
let me know.

Nope...no code when I disconnect the switch.

Will I have a G51MP in horz. postion that I will be working on.. will check a few things for you.

I took off the cold end collector box to insoect it. When I re assembled everything, the pressure switch pulled in and the unit is working fine for the last 24 hours.
Thanks for the manual. It helped having the expanded view.