SPN 171 FMI 2 is the Detroit DD Series diagnostic trouble code for ambient temperature sensor data erratic, and this page documents its official ambient air temperature sensor circuit troubleshooting sheet. The two-page sheet walks a technician through confirming the ambient air temperature sensor build, checking the Common Powertrain Controller (CPC) parameter group PRG031, and measuring the sensor supply voltage on the harness.
SPN 171 FMI 2 document details
| Brand | Detroit |
| Model coverage | DD Series — DD13, DD15, DD16 |
| System / group | DD Series (EPA 2007-Current) / DTCs |
| SPN | 171 |
| FMI | 2 |
| Fault | Ambient Temperature Sensor Data Erratic |
| Emissions / revision | EPA07, GHG14 — Copyright 2017 |
| Pages | 2 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code identity: SPN 171, FMI 2
- Ambient air temperature sensor equipped check
- CPC parameter group PRG031 configuration
- Harness supply voltage measurement (pins A and B)
- CPC connector #3 pins 2 and 15 repair path
Troubleshooting steps summarised
The procedure asks whether the vehicle is equipped with an ambient air temperature sensor and, if so, verifies that CPC parameter group PRG031 is configured correctly. The sensor is then disconnected, the ignition is turned ON (key ON, engine OFF), and the voltage between pins A and B on the harness side of the connector is measured. A reading between 4.5 and 5.2 volts calls for replacing the sensor, while a reading outside that band directs the technician to restore voltage between CPC connector #3 pins 2 and 15 and the harness pins A and B.
Applicable truck models covered
This sheet applies to Detroit DD Series engines (DD13, DD15 and DD16) built to EPA 2007 emissions with GHG14 hardware, and to Series 60 (DDEC VI) installations that share the Common Powertrain Controller. For engine specifications and warranty coverage, refer to the official Demand Detroit service resources. These troubleshooting steps are cross-checked with the Detroit Diesel Diagnostic Link (DDDL) and the official Demand Detroit service resources.
Who needs this SPN 171 FMI 2 guide
Diesel technicians and workshop owners diagnosing an erratic ambient air temperature reading, a mis-configured PRG031 parameter, or a suspect sensor supply circuit will use this guide before condemning parts.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download the complete step-by-step document.
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