A6 displayed, furnace may provide intermittent hot water or heating

Safety first: Isolate power before working on low-voltage sensor wiring.

• 1. Locate outdoor sensor: Usually mounted on an external north-facing wall away from direct sunlight.
• 2. Inspect cable: Check for cuts, UV damage, or rodent damage along the cable route.
• 3. Measure resistance: Disconnect the sensor and measure resistance at ambient temperature. Compare with Junkers NTC chart. Replace if out of range.
• 4. Wiring continuity: Verify continuity from sensor to boiler terminals. Repair any breaks.