Water pressure, the pilot light, and the safety switches boilers have that furnaces don't.
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Boiler problems
A boiler shares almost nothing with a furnace's airflow-based troubleshooting, because it is heating and circulating water rather than moving air: the checks here are specific to that.
Water pressure is the first gauge to learn
Most residential boilers display their water pressure on a simple gauge. And that single number rules in or out a whole category of common faults before you look at anything else. The guides here start with the checks that matter most when a boiler stops working, and with how to read a radiator that has gone cold.
Boiler guides
Cold at the top is air, cold at the bottom is sludge, cold all over is a valve: and a cold house is neither.
Power, the thermostat call and the safety lockout, in the order that costs you the least.
When the boiler runs and the house stays cold, the fault is in how the water moves.
A heater that sat idle for months often needs a setting, a switch or a gauge check, not a repair.
The thermostat clicks and nothing happens. Check the settings, the switch and the startup delay.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.