Gas appliances require a stricter DIY boundary than many electric appliances.
Start Here
Check user controls and obvious burner cleanliness.
Verify gas supply is intentionally on.
Observe ignition from a safe position.
Stop for persistent gas odor.
Use a qualified gas technician for valves, regulators and combustion adjustments.
The fastest way to get lost is to remove parts before you know which function failed. Gas appliances require a stricter DIY boundary than many electric appliances. For gas appliance boundaries, concentrate on temperature sensing, heating and safety controls. Use every working function as evidence: if one stage still behaves normally, that fact eliminates causes that would have stopped the appliance earlier in the sequence.
Observe the timing before touching anything. Note what happens immediately after Start, what changes just before the fault, and what remains normal afterward. Lights, relay clicks, water movement, motor or fan sounds, heat, vibration and error codes are useful only when tied to a point in the cycle. A fault that appears instantly is a different problem from one that appears only after the machine warms up or reaches a later stage.
Begin with the cheapest conditions to verify: check user controls and obvious burner cleanliness, then verify gas supply is intentionally on. Do not merely glance at them. Compare the actual condition with the manual, correct anything obviously wrong, and retest before moving deeper. Simple installation, maintenance and user-setting problems regularly imitate failed electrical parts.
If those checks pass, move to observe ignition from a safe position. Then decide whether stop for persistent gas odor can be evaluated safely with power isolated. Choose one test that answers one question: blocked or clear, open or closed, free or jammed, intact or electrically open. If a test result would not change your next decision, the test is probably not worth doing.
Avoid searching for leaks with a flame, altering jets or regulators, and continuing to ignite a burner after persistent gas smell. All three skip evidence and can turn a minor fault into an expensive one. After a correction, reassemble guards and connectors, restore normal settings, and reproduce the original operating conditions. A single successful start is not proof; the original symptom must stay gone through the stage that used to fail.
A Safe Step-by-Step Check
Document the exact symptom and pay special attention to check user controls and obvious burner cleanliness.
Remove external causes first; specifically verify verify gas supply is intentionally on.
With power isolated where required, inspect observe ignition from a safe position and then evaluate stop for persistent gas odor only if it can be done safely.
Reassemble fully and run a controlled test under the conditions that previously caused the fault.
Common Beginner Mistakes
Searching for leaks with a flame.
Altering jets or regulators.
Continuing to ignite a burner after persistent gas smell.
STOP AND GET HELP: If the next useful test requires exposed live mains, defeating a safety interlock, opening a sealed refrigeration circuit, disturbing gas controls, or entering a microwave high-voltage section, the diagnosis has reached the professional-service boundary.