Marey tankless heater shows E3 and stops heating; fan may run briefly then shut down.

Safety first: Turn OFF power and gas. Do not operate the heater if you suspect vent blockage or exhaust leakage—risk of carbon monoxide.

Check venting system:

• Inspect outside termination: Go outside and verify that the vent cap and intake (if concentric) are not blocked by leaves, snow, nests, or insect screens. Clear any obstructions.
• Check vent length and slope: Compare your installation to the Marey manual. Excessive horizontal runs, sagging pipes, or improper slope can cause condensation and backpressure. Correct any obvious installation errors.
• Inspect vent connections: With power OFF and cover removed, check that vent pipe connections at the heater are secure and not crushed or kinked.
• Examine flue gas sensor: Locate the flue/exhaust temperature sensor (usually clipped to the exhaust collar or inside the flue path). Ensure its wiring is intact and connector is firmly seated. If the sensor is cracked, corroded, or reading open/short (requires multimeter), replace it.

Test: After clearing obstructions and verifying sensor connections, restore power and gas and run hot water. If E3 persists, the flue sensor or control board may be faulty.

When to call a technician: For any vent system modification, flue sensor replacement, or if you suspect carbon monoxide issues.