Thermostat shows E4 and cannot control heating; app may indicate power supply or hardware fault.

Safety first: Turn off the circuit breaker feeding the thermostat. Confirm power is off before removing the unit.

Steps:

• Verify supply voltage: With the thermostat removed and power restored temporarily, measure voltage between L and N on the wall plate using a multimeter. It should match your mains (e.g., 100–240 V AC). If voltage is missing or unstable, correct the upstream wiring or breaker issue.
• Check terminal connections: Turn power off again. Inspect that the live (L) and neutral (N) conductors are fully inserted and clamped. Re-strip and re-terminate any damaged or oxidized wires.
• Confirm correct wiring diagram: Compare your connections to the Meross wiring diagram for your exact model (e.g., MSH300 or MTS100 series). Ensure no control output is mistakenly connected to neutral or earth.
• Test on a known-good circuit: If possible, temporarily connect the thermostat to a known-good mains feed on a bench or near the consumer unit. If E4 persists with correct voltage, the internal power supply is likely defective.

Outcome: If wiring and supply are correct but E4 remains, replace the thermostat main PCB or the entire thermostat unit.