Ruud U96VA0702317MSA Error Code 46: Low Pressure Switch Open, Inducer on Low Speed
What Does Code 46 Mean?
Code 46 on the Ruud U96VA0702317MSA is specific to two-stage operation. This furnace always ignites at high fire, then can step down to low-stage heating. Code 46 means that after it dropped to low stage — with the inducer at low speed — the low-pressure switch opened, so the required draft was not maintained. Because of that startup logic, this fault only appears after a successful high-fire light-off and the switch to low stage.
This is a critical fault. When it occurs, the gas valve is de-energized and the furnace attempts to restart after completing post-purge and blower off-delay. It is the low-speed sibling of code 45 (low-pressure switch open at high inducer speed) on the same switch.
Common causes include vent blockage or improper termination, vent length or elbows beyond the maximum, a faulty or disconnected inducer, split pressure-switch hoses, a faulty switch, and — notably for low-fire draft — a missing high-altitude kit at elevated installations, where thinner air makes low-speed draft marginal. This is a technician diagnosis.
What You'll Notice
- The furnace lights on high fire, then faults when it tries to run on low stage, showing code 46
- Heat may start then stop, with the furnace attempting to restart
- More likely at higher-altitude installations if the correct kit was not installed
- Cooling and fan still work
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Vent blockage or improper termination | Most common | ✗ Call a pro → |
| High altitude installation without proper kit | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms the fault occurs at the transition to low stage, then inspects the venting for blockage and proper termination and checks that length and elbow counts are within spec, since vent restriction is the most common cause.
For elevated installations they check whether the required high-altitude kit is installed, because reduced air density can leave low-speed draft too weak; they also check the inducer, hoses, and pressure switch, replacing whatever is at fault.
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:
- The furnace ignites but faults with code 46 when it tries to run on low stage
- The home is at a high elevation and you are unsure the correct high-altitude kit was installed
- The fault persists after the outdoor vent termination has been checked
Frequently Asked Questions
Why does code 46 only show up on low fire?
This furnace ignites at high fire and then steps down to low stage; code 46 is the low-pressure switch opening at the lower inducer speed, so it only appears after the switch to low-stage heating.
Could my altitude cause code 46?
Yes — at higher elevations thinner air weakens low-speed draft, so a missing high-altitude kit is a recognized cause and a technician can confirm whether the correct kit is needed.
✓ Verified against manufacturer service manual — March 2026