Carel AL26

What does Carel AL26 mean?

AL26 on the Carel Standard Roof-top controller (program +030221501 on pCO1/pCO3/pCOxs hardware) indicates that analogue probe B3 is faulty or disconnected. Probe B3 is one of the temperature or pressure measurement inputs used by the roof-top unit application. The alarm is manually reset and carries a 60-second detection delay. Any control or alarm function dependent on B3 is disabled while the fault is active.

Symptoms

• AL26 displayed on the pCO terminal with manual reset required.
• The B3 reading is absent or shows an out-of-range value on the status screens.
• Control functions driven by B3 — such as condenser pressure tracking or auxiliary temperature measurement — stop operating correctly.
• A 60-second delay elapses between the fault condition and alarm activation.

Common causes

• Probe B3 cable broken, disconnected at the sensor head, or loose at the pCO analogue input terminal.
• Probe B3 sensor element failed — resistance or voltage output outside the controller's configured measurement range.
• Wrong probe type wired to B3 vs. the input type configured in the controller parameters (e.g., NTC vs. 4–20 mA).
• Water or corrosion ingress on the probe terminal or cable connector.
• Cable routed alongside power or inductive wiring causing interference on the signal line.

Diagnostic steps

1. 01

   Identify what probe B3 measures on this unit

   Check the unit wiring diagram or commissioning records to confirm which sensor is connected to analogue input B3. Common assignments are condenser outlet air temperature, liquid line temperature, or a secondary circuit probe.

2. 02

   Inspect B3 terminal connections at the controller

   Power down the controller before working on signal wiring. Inspect the B3 input terminal for loose or corroded conductors. Re-seat and tighten the screw terminal.

3. 03

   Measure the probe output

   With power restored, measure the signal at the B3 terminal using a multimeter. Compare the reading against the expected output for the installed probe type at the current ambient temperature or pressure. An open circuit (OL) or zero reading confirms a wiring or probe failure.

4. 04

   Verify probe type configuration

   In the controller manufacturer parameter screens, confirm that the B3 input type parameter matches the physical probe installed. A misconfigured input reads a valid signal as a fault.

5. 05

   Replace the probe and reset AL26

   Install a compatible replacement probe of the correct type and range. After reconnection, press the alarm reset key on the pCO terminal to clear AL26 and confirm the B3 reading returns to a plausible value.

When to call a professional

Contact a qualified controls technician if AL26 persists after probe and wiring replacement, as this indicates a fault on the pCO analogue input circuit itself. If B3 monitors condenser pressure or a refrigerant temperature critical to safe system operation, do not run the unit in a degraded state without qualified assessment of the impact on system protection.