A Simple Fuse Causes A/C Failure

Sometimes, after finding the source of a problem, you tell yourself, “If I had taken a step back and looked at it from another angle, the solution would have been obvious.” And sometimes, taking a step back isn’t enough. You have to cross to the other side of the street to get a complete view of the situation.

A new customer came in two weeks before his vacation. “My air conditioning isn’t working, and I’d really like it fixed for our trip down South.” The pressure was on, and we immediately took it upon ourselves to do whatever it took to keep this new customer happy. Enter the vehicle: a 2021 Kia Telluride.

We brought the vehicle into the shop and started standard diagnostic checks on the A/C system. First, we confirmed the customer’s complaint, analyzed the refrigerant, and checked the charge levels, as 99% of air conditioning issues are caused by leaks. The refrigerant levels were extremely low, so we ran a nitrogen pressure test while attempting to run the compressor. It wouldn’t kick on. We connected our scan tool to command the compressor clutch directly, but got zero response. It was at this exact moment that a step back was needed, particularly for a vehicle we had never serviced before.

The Kia had a check engine light on. Logically, if there were a connection between the two issues, the client might have mentioned it. But there was no mention of the main warning light, so it must have been on for ages. WRONG.

Blowing the fuse

Several diagnostic trouble codes were stored in memory, but the one to keep in mind was P0597 — Thermostat Heater Control Circuit Open. On this vehicle, the thermostat has a built-in heating element that kicks in to open it up faster when needed.

Following the diagnostic tree for this code points right to a failed thermostat due to an internal short circuit, which is what popped the “Sensor 2” fuse. That single fuse powers a whole lineup of engine sensors, and also… the A/C compressor clutch.

So why didn’t the client make the connection? The thermostat failed during a season when air conditioning wasn’t needed, and his usual repair shop failed to figure out the problem, leaving the car as it was.

With the leak sealed and a fresh thermostat in place, the Telluride was able to hit the road right on schedule for vacation, with no check engine light on. As for us… well, we were able to move on to the next challenges.