Carrier 59SC5A Error Code 23: Pressure Switch Did Not Open
What Does Code 23 Mean?
Status code 23 on the Carrier 59SC5A means the pressure switch did not return to its open position when the control board expected it. The pressure switch is a normally-open safety device that closes only when the inducer motor pulls proper draft; once the inducer stops, the switch should drop back open. When the board sees it still closed at the wrong time, it flags code 23.
The most common cause is an obstruction in the pressure tubing, the narrow hose linking the pressure switch to the inducer housing or collector box. Trapped condensate, an insect nest, or debris can hold pressure on the switch diaphragm even after the inducer is off. The switch itself can also develop stuck contacts, more often in older units where the internal mechanism has worn.
This is the mirror image of code 31, where the pressure switch fails to close (or reopens) during the draft-proving stage. Code 23 is about the switch failing to open when draft stops. Both point to the pressure-proving system, but the fixes differ, and code 23 typically comes down to clearing the tubing or replacing a stuck switch, which is professional work.
What You'll Notice
- The furnace fails to start a normal heat cycle and code 23 is displayed
- The problem may appear after a period of heavy condensation or a long idle spell
- The furnace is more likely to show this on an older unit whose switch has aged
- The inducer may start but the ignition sequence does not proceed as expected
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Obstructed pressure tubing | Common | ✗ Call a pro → |
| Pressure switch stuck closed | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician isolates whether the fault is the tubing or the switch. The pressure tubing is inspected for trapped condensate, insect nests, or debris and blown clear if blocked, since an obstruction can hold the diaphragm closed. If the tubing is clear, the pressure switch is tested to confirm it opens and closes at its rated pressures; contacts that stay closed at zero draft mean the switch is mechanically stuck and must be replaced. Testing the switch against its rated values requires instruments and is not a homeowner step.
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:
- Code 23 is present, since verifying and clearing the pressure tubing or testing the switch requires professional tools
- The furnace will not complete an ignition sequence
- The unit is older and has a history of pressure-switch faults
Frequently Asked Questions
What does code 23 mean on a Carrier 59SC5A?
It means the pressure switch did not open when the inducer stopped and draft ended. Usually the pressure tubing is obstructed or the switch contacts are stuck closed.
Is code 23 the same as code 31?
No. Code 31 is the pressure switch failing to close (or reopening) during draft proving, typically a venting or draft problem. Code 23 is the switch failing to open when draft stops, typically obstructed tubing or a stuck switch.
Can I fix code 23 myself?
This one is best left to a technician. Confirming an obstruction and testing the switch against its rated pressures needs instruments, and a stuck switch has to be replaced.
✓ Verified against manufacturer service manual — March 2026