Autoterm / Planar AIR (2D/4D/8D/9D) error code 78
Flame failure during operation (PU-5: 3 blinks)
What to check, and in what order, is in the app. The flow Diesel Air Heater — Won't Start / Lockout (glow vs fuel) 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.
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- Autoterm AIR fault-code table + AIR Repair Manual + PU-5 Operation Manual v2025Autoterm
- Autoterm AIR User's Manual with installation instructions v12.2018-1 (PLANAR series)