Trane / American Standard Error 166.00: Low Superheat in Cooling, Indoor EEV (Meaning, Causes, Fixes)

What the code means

Trane's System Alert Guide lists ERR.166.00 as Low Superheat Occurrences in Cooling: the indoor EEV motor is at the minimum position but superheat remains less than 3 degrees for 20 continual minutes of cooling operation. Multiple cycles with low superheat result in a system lockout, and Trane's reset instruction is to cycle power. Trane's alert code table for its communicating air handlers (CL2.166.00, short text SYSTEM4 ERR, Indoor EEV error, severity Major) gives the same picture: the EEV is at the closed position and superheat is under 3 degrees, and five consecutive cycles with low superheat lock the system out. Superheat is how many degrees the refrigerant vapor has warmed above its boiling point by the time it leaves the coil. When it stays near zero with the valve closed, liquid refrigerant may be flooding through, which can wash back to the compressor, so the control shuts cooling down to protect it. The companion alert 166.01 (Normal severity) means the valve is closed and the coil is still flooding; Trane lists low airflow, refrigerant overcharge and a leaking check valve among the causes in that part of the table. Related alerts: 156 is the opposite problem (high superheat, often low charge) and 158 is indoor coil frost protection.

Likely causes

• The ET or GT temperature sensor on the indoor coil is out of calibration, so the control reads superheat wrong (Trane lists this first)
• An external check valve leaking refrigerant past the closed EEV (Trane notes not all models have an external check valve)
• The indoor EEV is stuck open or not seating properly on the valve body
• A restriction in the refrigerant circuit where the ET sensor is located; Trane suggests a coil frost test to find it
• Low indoor airflow, for example a dirty filter, a dirty coil or blower wheel, or closed registers
• Too much refrigerant in the system (overcharge), for example after a service visit that added charge

Safe DIY checks

1. Write down the exact alert (166.00 vs 166.01) and when it appears; Trane's table treats them differently.
2. Check the air filter and replace it if it's dirty. Low airflow across the indoor coil is one of the causes Trane lists, and it's the only one a homeowner can fix.
3. Make sure supply registers and return grilles are open and not blocked by furniture or rugs.
4. Look at the indoor coil area and the refrigerant lines for heavy frost or ice. If you see ice, turn cooling off and run the fan only until it melts, then call for service.
5. If the system has locked out, Trane's reset is to cycle power. Turn the system off at the thermostat, switch off power to the indoor and outdoor units for a minute, then turn it back on.
6. If 166.00 comes back after a filter change and a power cycle, stop resetting it. Repeated low-superheat running can push liquid refrigerant back to the compressor, and the remaining checks need gauges and Trane's diagnostic tools.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• 106.00 Trane HVAC System 106.00
• 126.00 Trane HVAC System 126.00
• 79 Trane HVAC System 79
• 178 Carrier HVAC System 178
• 44 Carrier HVAC System 44

All Trane codes →

Sources

1. Trane System Alert Guide (Aug 2020): ERR.166.00 Low Superheat Occurrences Cooling, indoor EEV at minimum but superheat below 3 degrees for 20 continual minutes, multiple cycles lock out, cycle power to reset; causes ET/GT sensor calibration, leaking external check valve, stuck EEV, restricted ET circuit (coil frost test); ERR.166.01
2. Trane alert codes table (communicating air handler, CL2 codes): CL2.166.00 Major, SYSTEM4 ERR, Indoor EEV error, five consecutive low-superheat cycles lock out; CL2.166.01 Normal, EEV closed and still flooding; low air flow, refrigerant overcharge, check valve leaking
3. Trane Florida Region Tech Tip (Dec 2024), Link Low Superheat Alert: update Hub to 8.01 or higher, charge mode cooling 5 minutes, indoor EEV open and closed tests, charge mode heating 5 minutes