I have a 350MAV that failed recently and need help proceeding forward. My problem is that I get the dreaded flash code 31 before the inducer motor ever starts up. Here is where I am at:

Procedure
1. Turn on 110VAV power
2. Set thermostat to heat and I see heat is active

Observations
1. Flash code 31 (3 short flashes/ 1 long) within a short period of time
2. Inducer motor does not start.
3. HPS has 24 VAC on one side and is open on the other.

I think the inducer motor is fine for if I cycle the 110VAC power it will run for a minute before shutting down. This is when the thermostat is active for heat at the time 110VAC is cycled. My guess is this is some kind of cool down cycle.

Any ideas what I should check next?

 

Look on the "cheat sheet" to determine what the flash code actually means.

 

It is supposed to mean a problem was detected using the HPS. The usual suspects are blocked intake/exhaust or the condensate drain. In my case, the inducer motor never starts up to detect a failure. I thought the HPS might have been detected as closed prior to the inducer starting up but was able to rule this out.

A new observation. I started debug on this around 10 AM this morning. The furnace appears to rerun the startup diagnostic periodically on its own. It appears to have just got a pass and the furnace started up (roughly 2:10 PM).

 

When the stat calls for heat the first thing that happens is the control board checks to make sure the pressure switches are open. Sounds like you have a pressure switch that is sticking.

 

Checking if the pressure switch was closed was my first guess. It's open. There must be another check that it does prior to starting the inducer motor.

 

Do you have 2 pressure switches ?

 

On a call for heat.... the draft inducer MUST start first.
Then the pressure sense closes.

With that furnace if you are getting an error code 31 AFTER the inducer is running.... you probably have a condensate drain issue.

 

skaggsje: There is only one pressure switch.

Pjmax: The code 31 occurs soon after the thermostat turns the heat on and the inducer never starts. I'm thinking the firmware checks a few things like making sure the pressure switch is open before powering up the inducer. It may also have a voltage sensor possibly.

I have seen the furnace CPU stop presenting the flash code and rerun the diagnostics several times while I was searching web for answers. I read somewhere that it does this periodically. The furnace actually started up and ran twice today but I was not near it when it happened so I am guessing it got through the diagnostics those times.

This furnace is 20 years old and in a terrible environment for electronics. It's in my non-insulated attic. The temperature around here gets up to 105 F often in the summer and has been high as 120 F. I've been debugging this guy on and off for a few weeks now. Initially, I found the fused 110 VAC power switch next to the unit had died after a few on/off cycles. I replaced it and have power now. It is now making me think that it is the controller board or AC transformer. There aren't a whole lot of other things left that could be bad for it to fail before the inducer starts.

 

I thought it useful to post a followup on my problem. A new controller board is on order and will arrive early next week. I did a little more research on the controller board (HK42FZ007) found it is very suspect for cracked solder joints. It was covered under:

"Bulletin United Carrier Corporation Technologies Carrier Residential Engineering SMB 96-0023 Subject: FURNACE CONTROL BOARD RESISTOR SOLDER JOINT FAILURES 04/17/96 SITUATION: A resistor solder joint failure mode has been identified on the following control boards used in the subject gas furnace."

The 350MAV units impacted by the tech bulletin had serial numbers starting with 3193A to 4595A. I am guessing this is week code/year. My 350MAV's serial number starts with 0495A so it is in the range.

We will see how the replacement goes next week.

 

The new control board came in today and I installed it. The furnace now works as good as new. The short description of my original problem was the furnace was posting flash code 31 without the inducer motor ever turning on. This would happen after furnace AC power was tuned on and then a call for heat was set at the thermostat.

 

Thanks for the update. You obviously have good problem solving skills!

On a side note--that model was involved in a class action suit many years ago (settled in 2007). Still on the original heat exchanger? Had it inspected lately?

 

All three of my furnaces were inspected three years ago. I had heard there was a class action suit on these furnaces but was not informed as to what the problem was. Do you have any specifics on the problem? Corrosion?

Back when these furnaces were first installed, the 90% efficiency units were pretty new to the market. The installer was not the greatest on earth. He did not run the condensate drains from the attic unit all the way to the outside of my home. I saw water and algae just below the skirt of my stucco and upon inspection in the crawl space found the lines stopped at the mud sill. I ran these outside twenty years ago.

A month ago I noticed water around my first floor furnace that is used the most. The side of the heater condensate drain line facing away from me was completely rotted out. He had plumbed it in copper. I re-plumbed it in PVC like what was done with the attic unit.

 

The heat exchangers are lined with plastic and apparently are more prone to early failure. Carrier extended the warranty out to 20 years. Mine finally failed at 23 with the symptom being water on the floor instead of in the condensate line.

 

Thanks guy46085. We set all three heaters and water heaters in drain pans. I didn't notice any water stains in the upstairs unit. My main unit is the one that supplies heat to the kitchen and family room. It had the corroded pipes that were replaced by me with PVC. I will monitor it since it too is out of extended warranty. Thanks again.