Ruud U96VA0702317MSA Error Code 22: Main Limit Open
What Does Code 22 Mean?
Code 22 on the Ruud U96VA0702317MSA indicates that the main limit switch has opened. This safety switch monitors the temperature inside the heat exchanger area. When the temperature exceeds a critical threshold, the limit opens to stop burner operation and prevent damage to the heat exchanger or surrounding components. Heating will not resume until the limit cools back to within normal parameters.
The main limit switch is a critical safety device. When airflow through the heat exchanger is insufficient, heat builds up inside the furnace instead of being distributed throughout the home. The limit switch detects this dangerous temperature rise and shuts down the burners. The blower motor typically continues to run to cool the heat exchanger down.
The most common reason for insufficient airflow is a dirty or clogged air filter. Other causes include blocked or closed supply registers, a failed or sluggish blower motor, or a dirty evaporator coil (in systems with central air conditioning). In rare cases, the limit switch itself may be faulty.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty air filter or blocked airflow | Most common | ✓ DIY fix → |
| Non-functional blower motor | Common | ✗ Call a pro → |
How to Fix It: Check and Replace the Air Filter
What You'll Need
- Replacement air filter (correct size for your system) 🛒 Find at FiltersFast · 🛒 Find at Amazon
- Flashlight
Steps
- Turn off power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the manual gas shutoff valve near the furnace to the OFF position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
- Locate and inspect the air filter The air filter on the U96VA is typically located at the return air opening on the bottom or side of the furnace cabinet. Slide the filter out and hold it up to a light source. If you cannot see light through the filter, it is too dirty and must be replaced.
- Replace the air filter Install a new filter of the correct size and type. Make sure the airflow direction arrow on the filter frame points toward the furnace (in the direction of airflow). A dirty filter is the most common cause of limit switch trips.
- Check all supply registers and return vents Walk through the home and make sure all supply registers and return air vents are open and unobstructed. Furniture, rugs, or curtains blocking vents can significantly reduce airflow through the system.
- Restore power and gas, then test Turn the gas supply valve to ON and flip the circuit breaker back to ON. Set the thermostat to call for heat and monitor the furnace for at least one full heating cycle. It may take a few minutes for the limit switch to cool and reset before the furnace will operate.
When to Call a Professional
Contact a licensed HVAC technician if:
- The limit switch continues to trip after replacing the filter and ensuring all vents are open
- The blower motor sounds unusually loud, runs at an abnormally slow speed, or does not start at all
- You suspect the evaporator coil (A/C coil) above the furnace may be dirty or frozen
- The furnace short-cycles repeatedly — runs briefly then shuts off
- The limit switch itself appears damaged or has tripped multiple times in a short period
Sources
✓ Verified against manufacturer service manual — March 2026