Lennox SLP98UHV Error Code E 310: Discharge Sensor Failure
What Does Code E 310 Mean?
The SLP98UHV includes a Discharge Air Temperature Sensor (DATS) that measures the temperature of the heated air leaving the furnace. The SureLight Variable Capacity Integrated Control uses that reading to help manage the modulating heat output and to keep an eye on supply-air temperature. E310 is set when the control sees the discharge sensor shorted or reporting a value outside its valid range.
A notable detail of this code is how it is triggered. E310 is activated during Field Test mode, and the control only reports it when the sensor is shorted or out of range — it does not flag an error if the sensor is simply disconnected. That behavior points toward an internal sensor failure or a shorted condition rather than a plainly unplugged wire, which helps a technician zero in on the sensor itself.
It is important to understand that E310 is a sensor fault, not an overheating event. It should not be confused with E 252 (discharge air temperature too high), which reports that the supply air has actually gotten too hot and usually points to restricted airflow or firing-rate issues. E310 instead means the temperature reading itself can no longer be trusted. The furnace may keep operating, but it loses the discharge-temperature monitoring the DATS provides, so the sensor should be replaced.
What You'll Notice
- The 7-segment display shows E 310, typically noticed during a diagnostic or Field Test check
- The furnace otherwise continues to heat
- The code appears when a technician runs Field Test mode rather than during normal daily use
- Discharge-air readouts on the control appear implausible or fixed
- No obvious change in comfort, since the furnace can still run without the sensor value
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Discharge air temperature sensor shorted or reading out of range | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms E310 is a sensor problem rather than a true over-temperature condition, because the response is completely different. Since the code is reported when the sensor is shorted or out of range, and not when it is merely disconnected, they inspect the DATS and its wiring and then compare the sensor's resistance to the temperature-versus-resistance reference for the model.
If the sensor reads shorted or its resistance does not track temperature correctly, it has failed and is replaced. The technician also verifies the reading is not being confused with an actual high-discharge-temperature situation, which would instead present as E 252 and call for checking airflow and firing rate rather than swapping the sensor.
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:
- E 310 appears during a diagnostic or Field Test check
- The discharge-temperature readout on the control looks implausible or stuck
- You want the sensor replaced at the next scheduled maintenance visit
- You are unsure whether the code is a sensor fault or an actual over-temperature condition
Frequently Asked Questions
Is E 310 dangerous, and can I keep using the furnace?
E 310 is a sensor fault, not an active overheating event, so the furnace may keep running. However, it loses the discharge-temperature monitoring the DATS provides, so the sensor should be replaced by a technician rather than left indefinitely.
Why does E 310 only seem to show up during a test?
This code is activated during Field Test mode, so it commonly surfaces when a technician runs diagnostics rather than during ordinary operation. The control also only reports it when the sensor is shorted or out of range, not when it is simply unplugged.
How is E 310 different from E 252?
E 310 means the discharge temperature sensor itself has failed or is reading out of range, so the reading cannot be trusted. E 252 means the discharge air really is too hot, which usually points to restricted airflow or firing-rate problems. One is a sensor replacement; the other is an airflow and combustion investigation.
✓ Verified against manufacturer service manual — March 2026