Safety first: Turn off power and gas. Do not operate the boiler with a suspected flue blockage. Carbon monoxide risk.

Steps:

Inspect venting: Check the entire flue run for sagging, disconnections, or obstructions. For condensing Peerless models, ensure condensate is draining and not pooling in the vent.
Check flue sensor: Locate the flue gas temperature sensor on the exhaust outlet. Inspect wiring and mounting. The sensor must be firmly attached to sense correctly.
Test sensor resistance: With power off, measure resistance and compare to Peerless specs. Replace if open/shorted.
Clean heat exchanger: If the boiler overheats the flue quickly, the heat exchanger may be fouled. A professional should remove the burner and clean the heat exchanger surfaces per Peerless instructions.
Reset and monitor: After repairs, reset the boiler and monitor flue temperature and combustion with proper instruments.

Boiler shows E8 and shuts down shortly after starting the fan

Possible Causes