Webasto Air Top EVO 40 / EVO 55 (substantially applies to Air Top 2000 STC) error code F18

What to check, and in what order, is in the app. The flow Diesel Forced-Air Heater — entry diagnostic (Espar / Webasto / Autoterm / Chinese) walks this code step by step with the pass/fail measurements, wiring schematics and procedures that go with it, and it works offline in the bay.