Sub-Zero Code 24: Defrost heater underheat

What 24 means

On legacy Sub-Zero 600 Series units like the 650/F and 690, and on 700 Series, 736 and IC-27 models including the 736TCI, code 24 means defrost underheat. The technician checks the defrost heater resistance and makes sure both the heater and the thermistor are mounted correctly. On the 648PRO, code 24 points to the defrost heater ohms in zone X, with X being 1, 3 or 4. The coil may not melt its frost fully, so cooling capacity drops over several days.

Common causes of 24

Defrost heater open or out of resistance range
Heater or thermistor not seated against the coil
Wiring break in the defrost heater circuit

✓ One safe check you can try

1) Note code 24 and, on a 648PRO, the zone number beside it. 2) Look for a white frost layer on the back panel of the freezer. 3) Keep doors closed so less humid air enters. 4) Never use a hair dryer or heat gun inside the cabinet. 5) Schedule service before the coil ices over completely.

! Why to stop there

The heater is under the evaporator cover and attached to the coil.
Resistance testing requires the unit unplugged and panels removed.
Incorrect mounting can cause repeat problems.

Opening a panel yourself does not automatically cancel a manufacturer warranty, but it can cause accidental damage, and some manufacturers set their own service conditions. Check your warranty terms, or ask us, before deciding.

→Same appliance

Other Refrigerator codes

23Sub-Zero 600, 700 and 648PRO Code 23: Defrost overheat with no feedback 50Sub-Zero 600 and 700 Series Code 50: Refrigerator compressor running too long 41Sub-Zero 648PRO Code 41: Zone compressor or wiring problem 81Sub-Zero 700-2 Code 81: Variable-Speed compressor fails to start 82Sub-Zero 700-2 Code 82: VS compressor controller needs replacement 88Sub-Zero 700TF Code 88: Board and compressor controller not talking 80Sub-Zero 700TF Code 80: No inverter feedback after a board change ddSub-Zero 600 and 700 Series Display dd: Diagnostics switched off

2424 questions

What does underheat mean?

The defrost heater did not warm the coil enough to clear the frost. The heater may be weak or open, or it may not be mounted tight against the coil where its heat is needed.

Can a loose heater clip cause code 24?

Yes. If the heater or its thermistor shifts, heat does not reach the coil evenly and the control reports underheat, even though the heater itself still works.

How do you handle code 24?

We measure heater ohms, check mounting and wiring, then replace or reseat parts. Labor is backed by our 30-day warranty, and you see the price in writing first.