Lennox Error Code 3: Pressure Switch Stuck Open
Warning: Your Lennox HVAC system has detected a pressure issue that requires professional attention.
LED Blink Pattern
LED blinks 3 times.
How to Fix Lennox Code 3
- Step 1: Check your air filter. If it's clogged, replace it with a new one.
- Step 2: If your system has a reset button, try pressing it to see if that clears the error.
- Step 3: If the error persists, do not attempt to fix it yourself. Call a professional HVAC technician.
How to Reset Lennox Code 3
- Turn off the furnace power switch, usually located on or near the unit.
- Turn off the circuit breaker for the furnace. Wait 30 seconds.
- Restore power at the breaker and flip the furnace switch back on.
- Set the thermostat to call for heat. If the error code returns after 2-3 ignition attempts, the fault requires repair.
Resetting clears the code from the display but does not fix the underlying fault. If the code returns after reset, proceed to the diagnostic steps above.
What Does Lennox Error Code 3 Mean?
Lennox error code 3 indicates that the pressure switch is stuck open. This switch is a safety component that monitors the system's pressure and prevents it from operating if there's a refrigerant leak. When it's stuck open, it's as if the system thinks there's a leak, even if there isn't. This can cause your system to lock out and not operate properly. This issue is typically caused by a faulty pressure switch or a problem with the system's pressure, such as a clogged filter or a refrigerant leak.
Common Causes
- Faulty pressure switch
- Clogged air filter
- Refrigerant leak
- Blocked condensate drain line
Diagnostic Steps
| If... | Check | Expected |
|---|---|---|
| The system is locked out and won't run | Measure the pressure switch voltage using a multimeter | 0 volts (open) or 24 volts (closed) |
| The pressure switch is stuck open | Inspect the air filter and replace it if necessary | Clean air filter |
Applicable Lennox Models
- Lennox Healthy Climate(dual_fuel)
- Dave Lennox Signature Collection(gas)
- EL296(electric)
- EL296V(electric)
- ELP(electric)
- Elite Series(electric)
- G61(gas)
- Health Saver Series(electric)
- ML195(dual_fuel)
- ML199(gas)
- NCB(electric)
- NPE(electric)
- Pace Series(electric)
- SLO(gas)
- SLP(electric)
- SLP98(gas)
- SLP98V(gas)
- Slimline Series(electric)
Replacement Parts
| Part | OEM Number | Est. Cost | Where to Buy |
|---|---|---|---|
| Pressure Switch | 40-000011-00 | — | Find Part |
Related Codes on Same Models
Other codes that affect the same Lennox model families as 3.
| Code | Description | Severity |
|---|---|---|
| 1 | Ignition Failure | Warning |
| 2 | Pressure Switch Stuck Closed | Warning |
| 4 | High Limit Switch Open | Warning |
| 5 | Flame Rollout Switch Open | Warning |
| 6 | Pressure Switch System Lockout | Warning |
| 7 | Ignition Lockout | Warning |
| 8 | Control Board Error | Warning |
| 9 | Line Voltage Polarity Incorrect | Warning |
| E250 | Lennox Code E250 - Indoor Blower Communication Error | Warning |