Refrigerator Not Cooling With Frost on the Freezer Wall — Heater or Thermistor?

A Whirlpool side-by-side stopped cooling and the rear freezer wall was solid with frost. Another company had already diagnosed it as a failed defrost heater. The heater tested fine. The real fault was the evaporator thermistor, and the repair came to $340.

Job details

Model: Whirlpool WRS315SNHM06
Symptom: Refrigerator not cooling, heavy frost on rear freezer wall
Suspected: Defrost heating element
Actual cause: Evaporator thermistor out of range
Fix: Manual defrost of the evaporator coil, thermistor replaced
Cost: $340
Location: Galaxy Way, Century City, CA 90067

What the client reported

Food in the fresh food section was going warm and the freezer was working harder than usual. When she opened the freezer and pulled things out, the whole back panel was covered in snow-like frost. A company had already been out, run a diagnostic, and quoted a defrost heater. She asked me to arrive with that part in hand.

Why the defrost heater is usually the wrong call

Frost on the rear freezer wall means the defrost cycle is not running. The heater is the part that does the melting, so it is the obvious suspect, and it is the part most techs name first. It also happens to be easy to sell.

The problem is that the heater is only one piece of the defrost chain. The control board decides when to run a defrost cycle, and it makes that decision using temperature data from the evaporator thermistor. If the thermistor reports a coil that is warmer than it really is, the board sees no reason to defrost. The heater never gets a signal. It sits there in perfect working order while ice keeps building on the coil until airflow stops and the fridge goes warm.

On this generation of Whirlpool side-by-side, the thermistor fails far more often than the heater does.

What it actually was

Before installing anything, I tested the heater in place. It had continuity and drew normal current, so it was not the failed component. I then checked the evaporator thermistor resistance against coil temperature. It was reading well out of range, which explains why no defrost cycle had run.

The repair

The evaporator coil was fully iced, so it had to come off slowly rather than with heat guns or scraping, which is how coils and aluminum tubing get damaged. Once the coil was clear, I removed the rear freezer panel, replaced the thermistor, and reassembled everything. About an hour on site. Temperatures pulled down normally the same visit.

What it cost

$340 total, parts and labor. Replacing the heater instead would have cost the client money and left the fridge frosting up again within a few weeks.

Frequently asked questions

Why is there snow on the back wall of my freezer?

The automatic defrost cycle is not running. Ice builds on the evaporator coil until air can no longer pass through it, and the fresh food section warms up first.

How do I know if it is the heater or the sensor?

The heater can be tested for continuity and current draw before any part is ordered. If it passes, the fault is elsewhere in the defrost chain, usually the thermistor or the control board.

Can I just unplug the fridge to melt the ice?

That restores cooling for a week or two, but the defrost cycle is still broken and the ice comes back.

Refrigerator problem in West Hollywood?

See refrigerator not cooling repair in West Hollywood or call 323-747-7098.