Carrier 58MVC Error Code 31: Medium-Heat Pressure Switch or HPSR Relay Did Not Close or Reopened
What Does Code 31 Mean?
Because the 58MVC modulates through low, medium, and high-heat stages, it uses more than one pressure switch to prove that draft is correct at each stage. Code 31 covers the medium-heat pressure switch input and the HPSR (high pressure switch relay). It is set when that input fails to close on a call for medium- or high-heat, or when it opens during medium- or high-heat operation.
Unlike the low-heat pressure switch faults (codes 32 and 43), which often come from a blocked vent or plugged condensate drain that a homeowner can check, code 31 is tied more directly to the control relay and gas-valve wiring. The manual states the control relay may be defective or the gas valve may be mis-wired. As the furnace steps up from low to medium or high heat, the board energizes the appropriate stage and expects the matching pressure proof; if a relay does not switch correctly or the valve is wired wrong, the medium-heat proof never appears or drops out.
The manual directs diagnosis to code 32 as well, because the low- and medium-heat pressure systems are related — a problem that shows on one stage can reveal itself on the other. Code 31 specifically flags the medium/high side and the HPSR relay circuit.
Testing the HPSR relay, confirming gas-valve wiring, and working inside the control and gas-valve circuits are professional tasks and are not homeowner-serviceable.
What You'll Notice
- The furnace runs on low-heat but fails or shuts down when it tries to step up to medium or high heat
- Short-cycling or a failed cycle when higher heat is called on cold days
- The furnace may seem to run only on its lowest stage
- Repeated failed starts when medium- or high-heat is demanded
- The problem may follow recent service or gas-valve wiring work
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Defective control relay | Common | ✗ Call a pro → |
| Gas valve mis-wired | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician verifies the medium-heat pressure switch and HPSR relay behavior as the furnace steps up in heat, distinguishing a relay that is not switching from a switch or draft problem. They test the control relay, inspect the gas-valve wiring against the diagram, and cross-check the low-heat side (code 32) since the two are related.
Depending on findings, the repair is a control board (relay) replacement or a gas-valve wiring correction. This work involves the control board and gas-valve circuit and is professional, not DIY.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- Contact a technician — a medium-heat pressure switch or HPSR relay fault points to a control relay or gas-valve wiring problem
- If the code followed recent service, have the gas-valve wiring verified against the diagram
- If the furnace only runs on low-heat and fails to step up, have the staging circuit inspected
- Do not attempt to service the control relay or gas-valve wiring yourself
Frequently Asked Questions
How is code 31 different from code 32 on my 58MVC?
Code 32 is the low-heat pressure switch, often caused by a blocked vent or drain you can check. Code 31 is the medium-heat pressure switch and HPSR relay, which points more toward a control relay or gas-valve wiring problem that needs a technician.
Why does my furnace run on low but fail on high heat?
Code 31 is set when the medium- or high-heat pressure proof does not close or drops out as the furnace steps up. That can leave the furnace unable to complete a higher-stage cycle, which a technician needs to diagnose.
Can I fix code 31 myself?
No. The likely causes — a defective control relay or a mis-wired gas valve — are inside the control and gas-valve circuits and require professional diagnosis and repair.
✓ Verified against manufacturer service manual — March 2026