Rheem R802VA07542117MSA Error Code *T085_F: Suction Line Temp Thermistor Failure (Two-Stage Only)
What Does Code *T085_F Mean?
Your two-stage, variable-speed R802VA07542117MSA carries sensors that reach beyond the furnace itself into the cooling side of the system. The suction line temperature thermistor measures the temperature of refrigerant returning to the compressor, and the Bluetooth Communicating IFC (Integrated Furnace Control) pairs that reading with the suction pressure to calculate superheat, which is how the electronic expansion valve (EXV) decides how much refrigerant to feed the cooling coil. Rheem lists T085_F as a two-stage-only fault, logged when this thermistor reads outside the valid range, usually from a failed sensor or a disconnected wire at the control.
Because this sensor serves the cooling system rather than combustion, furnace heating is not affected and the burner operates normally. Rheem rates T085_F as low severity. What can suffer is cooling: without a trustworthy suction-line temperature, the EXV cannot compute superheat accurately and may fall back to a default position, so air conditioning efficiency can drop even though heating is untouched.
T085_F is one of this board's cooling-system EXV superheat sensors, which work together: the suction line temperature (T085_F) and the suction pressure transducer (T086_F) are the two inputs to the superheat calculation, and T088_F flags when those combined EXV measurements are inconsistent. These are a different group from the air-temperature sensors (T081_F return, T082_F supply, T084_F outdoor) and from T087_F, the board's own temperature sensor. All are low-severity monitoring faults, not hard lockouts.
Because the suction line thermistor and the suction pressure transducer feed the same superheat math, a problem in one can show up alongside T086_F or trigger the combined-measurement code T088_F.
What You'll Notice
- The alphanumeric display shows "T085_F", or the LED blinks the code one digit at a time with a roughly three-second pause between digits
- Heating continues to work normally with no change at the registers
- Air conditioning may cool less efficiently or feel weaker than usual
- The fault stays logged in the Bluetooth contractor app after a power cycle
- No lockout occurs, and the problem may be most noticeable during cooling season
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failed suction line temperature thermistor | Most common | ✗ Call a pro → |
| Disconnected thermistor wiring at control board | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician locates the suction line thermistor clamped to the refrigerant suction line and its connector at the control, then measures the thermistor's resistance and compares it against the expected value for the line temperature. An open or out-of-range reading indicates a failed thermistor, while a good reading at the sensor but a bad one at the board points to a broken wire or a loose connector.
The technician also confirms the sensor is making good thermal contact with the line and checks its wiring against the suction pressure transducer, since both feed the EXV superheat calculation. This is informational only; refrigerant-side sensor work is not homeowner-safe and requires refrigeration certification.
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 code stays active and your cooling system is not performing as well as it should
- You also see T086_F or T088_F, pointing to a broader EXV superheat sensing problem
- Air conditioning efficiency or comfort has dropped since the code appeared
- You want the correct suction line thermistor part confirmed for this two-stage model before a repair
Frequently Asked Questions
Can I keep using my furnace with code T085_F showing?
Yes for heating. T085_F is a low-severity fault on the cooling side, so the furnace still heats normally. The sensor affects your air conditioning's EXV superheat control, so schedule a repair before or during cooling season rather than treating it as a heating emergency.
Why does a furnace code affect my air conditioning?
On this communicating system the furnace control also manages the cooling coil's electronic expansion valve. The suction line thermistor is a cooling-system sensor wired to the furnace board, so when it fails the furnace logs T085_F even though the impact is on cooling efficiency.
Is this the same as a low refrigerant charge?
Not necessarily. T085_F specifically flags that the suction line temperature reading is out of range, which usually means the sensor or its wiring, not the refrigerant charge. A technician checks the sensor first, then evaluates the refrigerant system if the reading turns out to be accurate.
✓ Verified against manufacturer service manual — March 2026