Bryant 350MAV Furnace Code 31 Failure - HVAC - DIY Chatroom Home Improvement Forum

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Old 10-10-2011, 02:07 AM   #1
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Bryant 350MAV Furnace Code 31 Failure

I have a Bryant 350MAV which would not start and exhibited the code 31 error. The unit is about 16 years old. I am posting my fix to this which may help some others and save some $'s. Here's my troubleshooting "path" and what I found. Hope it helps.

The furnace would not start, so I pulled out the documents which came with the furnace, found the troubleshooting guide and discovered the code 31 was being displayed.

After studying the schematic included with the furnace documents, I used a DVM to measure the voltage at each side of the signals on the pressure switch. When I turned up the thermostat to call for heat, 24volts AC would be applied to one side of the pressure switch. The inducer motor would start and after a few seconds, the pressure switch would close then showing 24v AC on the other side of the pressure switch. This indicated the inducer motor, the pressure switch, and the venting were all fine, thus eliminating most of the causes that are attributed to the error code 31 in the documentation and from online searches.

A few seconds later, the system would shut down because the HSI, Hot Surface Ignitor, would not start to glow as required for the next step in the heat starting sequence.

There is a "service and maintenance procedures" document which came with the furnace. In section 7, troubleshooting, it describes running a component test. This is initiated with a momentary short between the "TEST" quick-connect terminal and the COM terminal on the connection block on the control board in the furnace. It sequentially operates the inducer motor, hot surface ignitor, main blower at cooling speed and lastly, the main blower at heating speed. All of these operated perfectly when I performed the momentary short.

When rechecking the pressure switch, I noticed a soft buzzing sound from the control board area after the pressure switch would close. I realized this is likely the next relay, the HSIR, Hot Surface Ignitor Relay, attempting to correctly function to turn on the ignitor. The first thing I assumed was a faulty relay.

I pulled out the control board (original part number located on the board is 1012-83-941B which looks identical to other pictures for similar units in other threads on this site) and unsoldered the inducer relay and HSIR relay (use solder wick with good solder pencil fine tip). I was able to find the datasheets online for these particular relays. The expected resistance of the energizing coil inside each of these is listed on its datasheet and I was able to measure the relay's energizing coil resistance with the DVM resistance meter setting. These matched indicating the relays were fine.

At this point, I figured the control board may have a more serious problem. I decided to trace out the schematic for the circuits on the board starting with the inducer relay and HSIR relay. I traced out about half of the board, including all the relays and voltage regulation back to the two integrated circuits. The large integrated circuit is a micro-controller (a special purpose dedicated processor circuit) while the smaller one is a set of darlington transistors to drive the power needed to energize the relays. I ignored the other half of the board for now since if that were bad, it most likely would mean it was the main IC, the micro-controller, was bad and could only be fixed by replacing the entire board. The other parts were resistors (unlikely to fail unless over current, but no burn marks), two transistors (a possibility), and some other ceramic caps, zener diodes, and rectifying diodes (low likelihood and polarity testing looked good).

The HSIR and the GVR relays each have a capacitor in series between the darlington power transistor driver and the input coil of the relay. If these capacitors were to degrade, that can affect the timing and performance of those relays. Well, the HSIR relay is the one that would function during the "Component Test" sequence, but not during the actual furnace startup.

Checking the HSIR capacitor, this is a 47uF, 63v rating, electrolytic capacitor. Electrolytic capacitors are infamous for degrading over time, especially if the voltage rating does not provide sufficient voltage margin over the actual operating voltage usage, they become reversed biased during transient operation, they operate at elevated temperature, or just deteriorate with age. The GVR capacitor is a similar 47uF, 63v rating, electrolytic capacitor. They are side by side. The system runs on 24v AC from the transformer and is rectified to 24v DC for relay operation etc. I did not have an oscilloscope handy to look at the waveforms to see if the ratings could be exceeded or biasing reversed during the transient operation of the circuit.

In addition, there is only one other electrolytic capacitor on this board, a 22uF, 50v rating, component.

I went to a local electronics store and purchased two 47uF electrolytic capacitors rated to 250v (also the voltage rating that happened to be in stock). I also purchased a 22uF cap rated to 150v. The higher voltage rating provides a lot more operating voltage margin for dramatically improved reliability. These can also be purchased from online electronics distributors. Be sure not to change the capacitor values from their original as the values may be important for signal timing. I did not bother to work that out. Also, these parts are polarized, so they must be soldered in with the correct polarity, so keep track of how you pull the old ones out. There is a "-" minus sign also printed on the PC board indicating polarity in case you missed it.

Because the voltage ratings I chose were much higher, these parts were considerably larger than the ones I removed. The 47uF caps fit nicely in the housing because of where they are on the PC board. They do butt up against each other requiring a slight bend on the leads. The 22uF cap however does not fit standing upright in its location when placed back into the plastic housing. The choice is to put in a smaller and lower voltage rated cap or keep sufficient lead length to bend the cap over so it still fits in the plastic housing when you put it back together.

I put everything back together and turned on the unit. Works like new! The total cost was ~$3 for the three capacitors.

Good luck. Perhaps your failure and fix may be similar.


Last edited by maq0; 10-13-2011 at 01:14 PM. Reason: Fix typos
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