Carrier 58MVP Error Code Code 23: Invalid Blower Airflow Selection

What Does Code Code 23 Mean?

A code 23 on the Carrier HK42FZ indicates that the DIP switches controlling blower airflow selection are set to a combination the board doesn't recognize. The HK42FZ uses specific DIP switches to configure the blower motor speed for heating and cooling modes. If these switches are set to an invalid combination — for example, both high and low speed selected simultaneously, or a speed that doesn't exist on the installed blower motor — the board flags this error and will not operate.

This is a configuration error, not a hardware failure. It typically happens after a board replacement when the technician didn't transfer the DIP switch settings from the old board, or after someone accidentally changed a switch position. The airflow switches are separate from the general setup switches that cause a code 22 — code 23 is specifically about the blower speed configuration.

When this error is active, the furnace will not run heating or cooling cycles. The board needs a valid blower airflow setting before it will allow any operation, because running the blower at an incorrect speed could cause comfort issues, efficiency problems, or in extreme cases, safety concerns like heat exchanger overheating.

Common Causes

How to Fix It: Check and Correct Blower Airflow DIP Switches

What You'll Need

• Flashlight
• Small flat-head screwdriver or toothpick (to flip DIP switches)
• Furnace installation manual or wiring diagram (usually on a sticker inside the furnace cabinet door)
• Phone or camera (to photograph current switch positions before making changes)

Steps

1. Turn off power AND shut off gas supply Switch the furnace power off at the disconnect switch near the unit AND at the circuit breaker. Locate the gas shutoff valve on the gas line leading to the furnace and turn it to the OFF position (perpendicular to the pipe). If you smell gas at any point, leave the area immediately and call your gas company from outside.
2. Remove the furnace access panel Remove the lower front access panel to expose the HK42FZ control board. Set the panel aside carefully.
3. Locate the blower airflow DIP switches On the HK42FZ board, find the DIP switch bank. The switches that control blower airflow selection are typically a subset of the full switch bank. They are often labeled on the board or in the legend printed on the board itself. Common labels include 'HEAT SPD' and 'COOL SPD' or simply numbered positions that the manual maps to airflow settings.
4. Photograph the current switch positions Take a clear photo of all DIP switch positions before making any changes. This gives you a fallback if the new settings don't work.
5. Look up the correct airflow settings Check the installation instructions sticker inside the furnace cabinet door. It will have a table showing which DIP switch combinations correspond to which airflow (CFM) settings for heating and cooling. Match the airflow to your furnace's tonnage and ductwork. If your home has a 3-ton AC system, for example, you'd typically want the blower set to around 1200 CFM for cooling. The heating airflow is usually set lower. If you don't know your system's requirements, set the switches to the factory default position shown in the manual.
6. Adjust the blower airflow DIP switches Using a small flat-head screwdriver or toothpick, carefully set each airflow-related DIP switch to the correct position per the manual. Make sure each switch clicks fully into position — a switch left halfway will cause erratic behavior.
7. Restore power and test Replace the access panel. Turn the gas supply back on (handle parallel to the pipe). Restore power at the disconnect and breaker. Set your thermostat to call for heat and observe the startup sequence.

When to Call a Professional

Contact a licensed HVAC technician if:

• You cannot determine the correct blower airflow setting for your system (unknown tonnage, no manual available)
• The code 23 persists after correcting the DIP switches to the documented settings
• The blower runs but airflow feels extremely weak or extremely strong after adjustment
• You hear unusual noises from the blower motor after changing settings
• The DIP switches are damaged or unresponsive