Boiler displays E17; outdoor sensor reading is implausible or weather compensation not working.

Safety: Isolate power before working on wiring.

• Check sensor location: Ensure the CTC outdoor sensor is mounted on a suitable external wall, away from direct sunlight and heat sources.
• Inspect wiring: Follow the cable from the sensor back to the boiler. Look for damage, joints exposed to moisture, or loose terminal screws.
• Test sensor (technician): Disconnect the sensor at the boiler and measure resistance. Compare with CTC's temperature/resistance chart. Replace the outdoor sensor if readings are incorrect.
• Verify terminal connections: Ensure the sensor is connected to the correct low-voltage terminals on the boiler control board.