SPN 171 — Ambient air temperature
Safe to driveDrivable, but watch it yourself
Updated: 2026-11-14
J1939 code SPN 171. Below — how the fault is worded at the factory by Cummins ISB, Detroit Diesel, Dongfeng, Foton, International, KAMAZ, KAMAZ-5490, MAZ, Mercedes-Benz, Renault Trucks, SITRAK, YaMZ and ZF AS-Tronic, and what the second half of the code (FMI) means in the standard.
How to check it yourself
The outside air temperature sensor sits in the bumper, under the cab or in the mirror. Often it is read by the dash or the climate control, and the engine gets the value over CAN — then FMI 9 and 19 mean the message is lost, not that the sensor is faulty.
What it should read
| Resistance of a typical NTC sensor at 20 °C | about 2…3 kΩ |
| The same at 80 °C | about 300 Ω — more precisely per the factory table |
| Voltage at the harness connector with the sensor unplugged | about 5 V |
Check order
- FMI 9, 19 (no data on the bus): the code is not about the sensor. Check which unit transmits the temperature (dash or climate control) and its link — SPN 639.
- FMI 3, 4: unplug the connector and measure the sensor resistance. An open sensor or wire is seen by the ECU as −40 °C frost (voltage high, FMI 3), a short as severe overheating (FMI 4). With the connector unplugged, the harness contacts should show about 5 V.
- Compare with a thermometer in the shade. A sensor at the bumper in the sun, or near a hot engine, reads wrong when parked and evens out on the move — this is not a fault.
Common causes — from most to least frequent
- Sensor broken by a stone or torn off in the bumper.
- Corroded connector: the area is wet and salty in winter.
- No link to the unit that transmits the value.
- The sensor itself.
Cummins ISBDetroit DieselDongfengFotonInternationalKAMAZKAMAZ-5490MAZMercedes-BenzRenault TrucksSITRAKYaMZZF AS-Tronic
Cummins ISB 6.7 engine
| FMI 2 | Ambient air temperature signal erratic or incorrect |
| FMI 3 | Ambient air temperature sensor circuit voltage high |
| FMI 4 | Ambient air temperature sensor circuit voltage low |
| FMI 9 | Ambient air temperature abnormal update rate |
| FMI 19 | Ambient air temperature network data error |
Detroit Diesel
| FMI 9 | DD15: Ambient Air Temperature Signal Not Available Via the Controller Area Network; DD8: Ambient Air Temperature Sensor Data Message Stopped Arriving |
| FMI 14 | DD15: Ambient Air Temperature Signal Not Available Via the Controller Area Network; DD8: Ambient Air Temp Sensor Data Not Received This Ignition Cycle |
| FMI 19 | DD15: Ambient Air Temperature Signal Error; DD8: AMB Message (Ambient Temperature) – Missing / Not Available |
Dongfeng
| FMI 2 | Incorrect ambient temperature signal. Scan tool code: P0071. Effect: Fan and warm-up control may misbehave. |
| FMI 3 | Short circuit to power or open circuit of the ambient temperature sensor. Scan tool code: P0073. |
| FMI 4 | Short circuit to ground of the ambient temperature sensor. Scan tool code: P0072. |
Foton
| FMI 3 | High intake air temperature. Scan tool code: P0073. |
| FMI 4 | Air temperature sensor fault. Scan tool code: P0072. |
International
| FMI 3 | Ambient Air Temperature — Fault on Analog Input 3 above normal when used for outside temperature |
| FMI 4 | Ambient Air Temperature — Fault on Analog Input 3 below normal when used for outside temperature |
KAMAZ engine (J1939)
| FMI 0 | Ambient air temperature sensor: physical range limit |
| FMI 1 | Ambient air temperature sensor: physical range limit |
| FMI 2 | Ambient air temperature. |
| FMI 3 | Ambient air temperature sensor. |
| FMI 4 | Ambient air temperature sensor. |
| FMI 5 | Ambient air temperature sensor circuit open |
| FMI 7 | Ambient air temperature sensor signal out of range |
| FMI 9 | Ambient air temperature. |
| FMI 12 | Ambient air temperature sensor |
| FMI 19 | Ambient air temperature. |
KAMAZ-5490 (Mercedes, K-line)
FMI is per the block maker's own table, not the J1939 table below.
| FMI 2 | Ambient air temperature: sensor circuit range / performance. K-line code on KamAZ-5490: MR2 13617. |
| FMI 3 | Ambient air temperature sensor: voltage too high. K-line code on KamAZ-5490: ADM3 16603. |
| FMI 4 | Ambient air temperature sensor: voltage too low. K-line code on KamAZ-5490: ADM3 16604. |
| FMI 9 | Ambient air temperature: CAN message update rate abnormal (ADM3 16609); Ambient air temperature: CAN message lost (MR2 13604). K-line code on KamAZ-5490. |
| FMI 19 | Ambient air temperature: erroneous data received over CAN. K-line code on KamAZ-5490: ADM3 16619. |
MAZ
| FMI 3 | Ambient air temperature sensor fault - Voltage above allowed. Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
| FMI 4 | Ambient air temperature sensor fault - Voltage below allowed. Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
| FMI 12 | Ambient air temperature sensor fault - CAN bus signal implausible. Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
Mercedes-Benz
| FMI 2 | Ambient Temperature Sensor Data Erratic |
| FMI 3 | Ambient Temperature Sensor shorted to Ubat |
| FMI 4 | Ambient Temperature Sensor shorted to GND |
| FMI 9 | J1587 Ambient Air Temp Sensor Data Message Stopped Arriving |
| FMI 14 | J1587 Ambient Air Temp Sensor Data Not Received |
Renault Trucks
| FMI 8 | Erroneous “actual engine torque” signal. Repair: Run the diagnostic check of the EECU. Check the programming. Switch the ignition off, switch it on again and repeat the test. If the fault persists, replace the control unit. |
SITRAK
| FMI 2 | Implausible ambient air temperature sensor signal. Scan tool code: P0071. Effect: Fan and warm-up control may misbehave. |
| FMI 3 | Ambient air temperature sensor voltage above the upper limit. Scan tool code: P0073. |
| FMI 4 | Ambient air temperature sensor voltage below the lower limit. Scan tool code: P0072. |
| FMI 12 | Ambient air temperature CAN message error. Scan tool code: P0070. Effect: Loss of communication with the modules, some functions are disabled. |
| FMI 16 | Ambient air temperature above the upper limit. Scan tool code: P0070. Effect: Fan and warm-up control may misbehave. |
| FMI 18 | Ambient air temperature below the lower limit. Scan tool code: P0070. Effect: Fan and warm-up control may misbehave. |
| FMI 23 | Disturbed transmission of the ambient air temperature over CAN. Scan tool code: P0070. Effect: Loss of communication with the modules, some functions are disabled. |
YaMZ
| FMI 3 | Ambient air temperature sensor fault (Voltage above allowable limit). Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
| FMI 4 | Ambient air temperature sensor fault (Voltage below allowable limit). Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
| FMI 12 | Ambient air temperature sensor fault (CAN bus signal implausible). Blink code: 2-3-5. Repair: Check the ambient air temperature sensor and its wiring. |
ZF AS-Tronic gearbox
FMI is per the block maker's own table, not the J1939 table below.
| FMI 8 | No CAN bus signal: "engine torque". |
What FMI means in J1939
| SPN 171 FMI 0171/0 · 171.0 | data above normal range (most severe level) |
| SPN 171 FMI 1171/1 · 171.1 | data below normal range (most severe level) |
| SPN 171 FMI 2171/2 · 171.2 | data erratic or intermittent |
| SPN 171 FMI 3171/3 · 171.3 | voltage above normal or shorted high |
| SPN 171 FMI 4171/4 · 171.4 | voltage below normal or shorted to ground |
| SPN 171 FMI 5171/5 · 171.5 | current below normal or open circuit |
| SPN 171 FMI 7171/7 · 171.7 | mechanical system not responding |
| SPN 171 FMI 8171/8 · 171.8 | abnormal frequency or pulse width |
| SPN 171 FMI 9171/9 · 171.9 | abnormal update rate |
| SPN 171 FMI 12171/12 · 171.12 | faulty device or component |
| SPN 171 FMI 14171/14 · 171.14 | manufacturer-specific instructions |
| SPN 171 FMI 16171/16 · 171.16 | above normal — moderate severity |
| SPN 171 FMI 18171/18 · 171.18 | below normal — moderate severity |
| SPN 171 FMI 19171/19 · 171.19 | error in received network data |
Can I keep driving
Safe to drive. Below — how much time you have and what happens if you keep going.
What happens if you keep driving
Time you have: Drivable, but watch it yourselfThis is an electrical fault: an open, a short or a bad contact in the sensor circuit. The value it measures is most likely fine.
You can drive. But until the circuit is repaired this system runs blind and cannot warn you about a real failure.
Nearby: other
- SPN 146Tire Pressure, Steer Axle Circuit
- SPN 149Engine Intake Air Mass Flow
- SPN 150Front range group stuck in an intermediate position
- SPN 152ECU Reset Count
- SPN 165Compass Heading
- SPN 220
- SPN 253
- SPN 275
- SPN 276
- SPN 395
Scanner threw several codes at once? Paste the whole list — we will show which one is the root cause and which ones simply followed.