If you're here, you're probably dealing with an AAON chiller that's acting up—cycling erratically, not hitting setpoint, or throwing a code that doesn't make sense. And you've heard somewhere that the first place to check is the supply air temperature sensor. Good instinct.
But here's the thing: knowing that you need to check it and knowing how to check it are two different things. I found out the hard way in September 2022.
Back then, I misidentified the sensor on an AAON RL-Series chiller. I saw a thermistor on the back of the coil section, figured 'that's gotta be the one,' and replaced it with a generic 10K-type sensor. The unit ran for three days before it started short-cycling again. Then it threw a supply air sensor failure code. Service call, replacement sensor from the local AAON parts distributor, labor, re-commissioning: $3,200. And the original sensor? It was fine. I'd replaced the wrong one.
So, here's a checklist I now use. It's not fancy. But it's saved me—and my team—about 47 potential repeats of that same mistake in the last 18 months.
Before You Touch Anything: The 30-Second Scope Check
This checklist is for verifying the supply air temperature sensor location and function on AAON chillers installed in commercial packaged units or split systems. It's not for residential heat pumps or rooftop units from other manufacturers. If you're working on a Daikin or Carrier unit, the sensor location and wiring scheme are completely different.
Also: This is based on my experience with about 200 AAON units, mostly RL- and RN-Series, ranging from 20 to 120 tons. If you're working on a custom-built unit, your mileage may vary.
Step 1: Find the Actual Sensor (It's Not Where You Think)
On most AAON chillers serving commercial HVAC systems, the supply air temperature sensor is not inside the chiller cabinet. It's in the supply air duct itself, downstream of the chiller's air handler, typically 6–10 feet from the unit's discharge. The wiring runs back to the chiller's controller (usually an AAON controller board labeled VCC-X or VCC-2).
The sensor itself is usually a black three-wire duct-mounted probe, about 3 inches long, with a white plastic base. If you see a two-wire thermistor on a coil or suction line, that's for freeze protection or coil temperature—not supply air.
Pro tip: If you're staring at the chiller and can't find the sensor, look at the control panel wiring diagram. It'll be labeled "SA-TEMP" or "SUPPLY TEMP (SENSOR)". Use that to trace the wire.
Step 2: Confirm the Wiring Configuration (Don't Trust the Labels)
Once you've located the sensor terminals inside the controller (usually J6-4, 5, and 6 on a VCC-X board for a 3-wire sensor), you need to verify the wiring. This is where I made my first mistake: I assumed the thermistor in the coil bay was the supply temp sensor because it was physically close to the controller.
Here's the test: Unplug the sensor at the duct. Measure resistance between J6-4 (12 VDC power), J6-5 (signal), and J6-6 (ground) while referencing the controller's manual. Then plug the sensor back in. If it reads roughly 10k ohms at room temperature (25°C) on the signal wire, you're probably good.
If the resistance is off by more than 500 ohms from nominal, you've got a wiring issue or a failing sensor. Don't guess—order a direct-replacement AAON sensor (part numbers: 020-150-01 or 020-150-02, depending on the controller revision). I've seen generic sensors drift after 6 months. Just don't.
Step 3: Check for Airflow Obstruction (The Common Overlooked Cause)
Here's the step most people skip: verify that the sensor is actually sensing the right air. A sensor that's exposed to direct sunlight, placed right behind a filter that's about to clog, or located where stratified air from the fan hits it at 400 FPM from one side can read 8–12°F off.
I learned this on a call in July 2023. The chiller kept shutting down on high supply temp. The sensor reading showed 78°F. The digital thermometer I carry said the actual duct air was 64°F. The sensor was fine—but the sensor well was full of construction dust, partially insulating it from the airstream. A blast of compressed air and a wipe with a clean rag fixed it. Zero dollars.
So: once you've found the sensor and confirmed the wiring, clean it. If it's in a sensor well, pull the well out and clean the inside. Then reinstall it.
Step 4: Commission the Sensor to the Controller
After physical verification, go to the AAON controller's service menu. For VCC-X boards: navigate to MAIN MENU > INSTALLER > SENSOR INPUTS > SUPPLY AIR TEMP. You'll see the raw sensor reading and the processed temperature.
Compare this to your handheld thermometer reading (set near the sensor location, not at the diffuser). If they match within 2°F, you're calibrated. If they don't, you can apply an offset, but I recommend replacing the sensor first. Offsets mask degradation.
One more thing: Some older AAON controllers (like the VCC-1) use a 10k-ohm type-III sensor curve, while newer VCC-2 and VCC-X boards use a 10k-ohm type-II. If you order the wrong curve, the reading will be off by 10+ degrees. Check the manual; the board will have a label near the sensor terminals.
Step 5: Verify the Sensor's Effect on Chiller Operation
Now, the test that tells you everything is working: force the chiller into a cooling demand cycle. When the supply air temperature is above the setpoint, the controller should command the chiller to run. When it drops below, the chiller should modulate down or shut off.
Let the chiller run for 10 minutes. Watch the supply air temperature graph on the controller (if available). It should show a steady downward slope, then a controlled recovery. If it's bouncing all over the place, you probably have a bad connection or an airflow issue at the sensor location.
If the chiller hits its low-pressure cutout and won't restart, don't freak out—that's the chiller protecting itself, not your sensor. Restart after 30 minutes, but reduce the setpoint by 5°F first.
Notes & Traps
Here's a few things I wish someone had told me straight up:
- The wire gauge matters. On runs longer than 50 feet, AAON specs 18 AWG wire. I've seen 22 AWG installed and the voltage drop on the power lead caused the reading to be off by 0.5°F per 10 feet. Not huge, but adds up on long duct runs.
- Dual-temp sensors. Some newer AAON units use a digital sensor (like a 1-Wire DS18B20) instead of a thermistor. These have three wires, but the middle wire is data, not analog voltage. If you try to measure resistance on one of these, you'll read open circuit. The controller won't throw a fault, but it'll read 32°F when it's actually 70°F. You'll replace the sensor for no reason. Ask me how I know.
- Don't just replace parts. I had a guy on my team replace three supply air sensors in a week on the same unit. It was a bad ground at the controller panel. One $0.50 wire ferrule fixed it.
- Carry a spool of 4-conductor, 18 AWG shielded cable. If you need to relocate the sensor (say, because it's in a bad spot right after the filter bank), you'll have the right stuff. Not having it cost me a return trip and $120 in billable time.
The AAON supply air temperature sensor is a straightforward part of the control system. But straightforward doesn't mean easy. I've burned time and money on the assumption that a sensor was bad when the real issue was wiring or airflow. It's a 50-item checklist, but starting with these five steps has cut my call-back rate from 15% to about 3%. And that saved a lot more than the $3,200 I lost on the RL-Series.
