Boiler shows E3 and will not start, fan may run briefly

Safety first: Turn off power and gas. Allow the boiler and venting to cool before inspection.

• Step 1 – Inspect venting: Follow the exhaust vent from the boiler to the termination. Look for kinks, sags, disconnections, or signs of crushing. Outside, check for obstructions such as snow, ice, insect screens clogged with lint, or animal nests. Clear any blockage.
• Step 2 – Check air intake: For direct-vent Rheem boilers, inspect the intake pipe for debris or blockages. Ensure the intake terminal is not too close to other exhaust outlets or corners that can cause recirculation.
• Step 3 – Reset and test: Restore power and gas, then reset the boiler. If E3 clears and the unit runs normally, monitor operation to ensure the fault does not recur.
• Step 4 – Test flue temperature sensor: If E3 returns with clear venting, the flue thermistor may be faulty. With power off, disconnect the flue sensor from the main PCB and measure its resistance. Compare to Rheem’s temperature/resistance chart. Replace the sensor if readings are out of range.
• Step 5 – Check combustion settings: If the boiler is overfiring, flue temperatures can exceed limits. A qualified technician should verify gas input, combustion analysis, and fan speed settings per Rheem specifications.

Warning: Never operate the boiler with a known vent blockage or bypass any safety sensor. Carbon monoxide poisoning and fire hazards are possible.