Bosch
ATL
ProtectAmbient temperature limited
Outdoor T4 outside the allowed window (cool: T4 ≥125°F or <15°F; heat: T4 ≥86°F or <−4°F on IDS 2.0), or the reversing-valve call is wrong so the outdoor thinks it is in the wrong mode.
Walk ATL in Diagnose
Sign in for the next measurement
First check
1 Confirm O/B is energized the way THIS outdoor expects
Most US splits energize O in cool. Some Rheem/Ruud energize B in heat. Read the Bosch BOVA 2.0 diagram, not a generic heat-pump card. Idle ≥5 min, compare T4 on the board to real outdoor temp (10K sensor, ~10 kΩ at 77°F).
Expect: Correct 24 V at the solenoid for the mode that is failing.
The rest of the walkthrough is Diagnose.
On the plates
- Idle ≥5 min, compare T4 on the board to real outdoor temp (10K sensor, ~10 kΩ at 77°F).
- Heat call must energize B per the Bosch diagram. Y-only in heat, or B stuck on in cool, produces ATL.
- 18–28 VAC B–C at the outdoor during a heat call.