Ako E21 Error Code

What does Ako E21 mean?

E21 is the second-evaporator equivalent of E11 but references probe S3. It indicates that probes S1 (cold-room) and S3 (second evaporator) are reading similar temperatures — insufficient differential for the SELFDRIVE system on the second evaporator. The SELFDRIVE calibration for the second evaporator is suspended and the controller falls back to standard regulation for that circuit.

Symptoms

• E21 appears on the display.
• SELFDRIVE calibration for the second evaporator using S3 cannot proceed.
• Controller uses standard regulation for the second evaporator.
• Alarm relay activates.
• Temperature differential between S1 and S3 is smaller than required for SELFDRIVE.

Common causes

• Probe S3 is incorrectly positioned — reading air temperature close to S1 rather than cold evaporator coil temperature (most common).
• Second evaporator is heavily iced, insulating S3 from the coil.
• S1 and S3 probes have been swapped during installation or a maintenance visit.
• Refrigerant shortage on the second evaporator circuit causing the coil to warm toward cold-room temperature.
• Blocked airflow over the second evaporator.

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Diagnostic steps

1. Verify probe S3 is properly attached to the second evaporator coil

   Confirm S3 is physically clipped to the coldest accessible point on the second evaporator coil and is not hanging in free air or resting on a frost surface.

2. Confirm S1 and S3 are not swapped

   Trace cables from both probes to their controller terminals. S1 is the cold-room air probe and S3 is the second evaporator coil probe. Re-wire if swapped.

3. Inspect second evaporator for ice blockage

   Heavy ice on the second evaporator can insulate S3 from the coil metal. Initiate a manual defrost on the second evaporator circuit and recheck the S1–S3 temperature differential.

4. Monitor second evaporator refrigerant performance

   Check the suction line temperature at the second evaporator. A warm suction line with a small S1–S3 differential indicates refrigerant shortage or a failed expansion valve on that circuit.

5. Restart SELFDRIVE calibration

   After correcting probe placement and clearing ice, follow the AKO manual to restart SELFDRIVE calibration for the second evaporator. Verify E21 clears.

When to call a professional

Call a licensed refrigeration technician if E21 persists after correcting probe positions and performing a manual defrost, as an insufficient S1–S3 temperature differential typically indicates a refrigerant system issue on the second evaporator circuit requiring pressure measurements and potentially refrigerant charge adjustment.