Code A7 = Outlet thermistor fault. The temperature sensor at the hot-water outlet of the heat exchanger is reporting an out-of-range value (either open circuit or short circuit).
Diagnostic sequence
1. Sensor wiring
Check the thermistor wiring at the PCB. Loose connector or damaged wire is the most common cause. Re-seat the connector.
2. Sensor itself
The thermistor is a small temperature-sensing probe in the outlet pipe. If failed, replace ($25-60 OEM, dealer-installed).
3. Water-side blockage
Heavy scale on the thermistor probe can cause erratic readings. Descaling the unit may resolve.
Reset procedure
- Power cycle the unit
- If code returns, escalate to wiring inspection
- If wiring is fine, replace the thermistor
Bottom line
Code A7 is a sensor or wiring fault. Often resolves with reset + connector re-seat. Persistent code = sensor replacement.