SPN 171/FMI 20: Ambient Air Temperature Sensor Fault

What’s Going On Here

Every time you shut your truck off, the engine slowly cools down until it matches the outside air. The truck’s computer, called the Motor Control Module, keeps track of this cool-down process closely. After the engine has been sitting off for a full 8 hours, it compares the ambient air reading it’s getting against what it actually expects. If that reading is off by more than 20°C (36°F), this code gets stored.

In plain words, the truck feels like the outside temperature sensor is telling it something that doesn’t line up, given how long the truck has been parked and cooling.

When The Truck Runs This Check

This isn’t something checked all the time. The truck only runs this specific test when all of these line up together:

Condition	Required Value
Ignition	Key ON, engine OFF
Battery Voltage	Above 11 volts
Engine Speed	Zero RPM
Outside Temperature	Below 40°C (104°F)
Cool Down Time	At least 8 hours

Once every box is checked, the truck runs the test one time, and it wraps up in about 5 seconds. This happens once per cool-down cycle — basically, once every time the truck sits long enough to fully cool off.

What You’ll See on the Dash

If this fault trips, you’ll likely see your check engine light come on. That’s really the only sign — your truck won’t drive differently, and there’s no dip in power tied to this fault.

First Thing Worth Checking

Before touching any wires or sensors, ask one simple question: has the truck’s main computer been reprogrammed recently, or has its internal clock been changed or reset? A clock change alone can throw off how the system tracks the 8-hour cool-down window and trigger this code by itself. If nothing like that happened, move on to the next check.

Rule Out Other Related Codes

Plug in your diagnostic tool and turn the key to ON without starting the engine. Look through the other codes stored. If SPN 171 with FMI 3 or FMI 4 shows up too, deal with those first, since they often point to the real root cause and can trigger this code as a side effect. If those aren’t present, move ahead to checking the sensor itself.

Look At The Sensor Wiring

Unplug the connector going to the ambient air temperature sensor and inspect it closely. You’re checking for anything corroded, greenish, cracked, or generally beat up. Your truck’s manufacturer literature will show exactly where this sensor sits if you’re unsure.

If you spot damage or corrosion, clean or repair the connector, then check if the fault comes back. If everything looks clean, the sensor itself has likely gone bad internally and needs to be replaced. Your OEM service manual will walk you through the exact removal and installation steps.

Confirming You Fixed It

Once a repair is done, take the truck for a 30-minute drive and let the coolant reach normal operating temperature. Then check that the outside air reading is within about 10°C (18°F) of the compressor inlet temperature reading. If those two line up closely, the repair worked and the sensor is reading accurately again.

A Quick Reference Detail

For anyone pulling parts or logging the repair, this sensor is commonly filed under component code 045-021-006, listed as the Ambient Air Temperature Sensor.