Thermostat displays E18 and may show a persistent low battery or power error even when C-wire is present.

Safety first: Turn off power to the HVAC system at the breaker before testing wiring.

• Step 1 – Check voltage at thermostat: With power on and caution, measure AC voltage between R and C at the thermostat. It should be around 24 VAC. If significantly lower, suspect wiring or transformer issues.
• Step 2 – Inspect connections: Tighten R and C terminals at both thermostat and control board. Look for corroded or poorly crimped splices.
• Step 3 – Reduce load: If multiple accessories (humidifier, dehumidifier, ERV) share the same transformer, consider disconnecting one temporarily to see if E18 clears.
• Step 4 – Check control board transformer: At the Carrier control board, verify 24 VAC output is stable under load. A weak transformer may need replacement.
• Step 5 – Replace thermostat: If supply voltage is good but E18 persists, the thermostat's internal power circuitry may be defective.
• When to call a pro: Transformer replacement and load calculations are best handled by a technician.