NOx Sensors (Upstream & Downstream)
Location, connector pinout, test procedure, and replacement guide for upstream and downstream NOx sensors on Class 8 diesel trucks.
Last updated: 2026-09-20
Overview
NOx sensors measure nitrogen oxide concentration in the exhaust. The ECM uses upstream NOx (sensor 1) and downstream NOx (sensor 2) readings to calculate SCR conversion efficiency. A failed NOx sensor causes false SCR efficiency readings and can trigger DEF inducement.
Sensor Locations by Platform
Cummins X15
Upstream (Sensor 1): Threaded into exhaust pipe before SCR catalyst, after DPF. Downstream (Sensor 2): Threaded into tailpipe after SCR catalyst.
X15 NOx sensors use CAN communication. Sensor replacement requires ECM reset.
PACCAR MX-13
Upstream (Sensor 1): Passenger side exhaust, between DPF outlet and SCR inlet. Downstream (Sensor 2): After SCR catalyst.
MX-13 NOx sensor replacement requires DAVIE sensor reset procedure.
Detroit DD15
Upstream (Sensor 1): Passenger side, threaded into decomp tube upstream of SCR brick. Downstream (Sensor 2): After SCR catalyst in tailpipe.
DD15 NOx sensors are part of the ACM network. DDDL required for sensor reset.
Volvo/Mack MP8
Upstream (Sensor 1): After DPF, before SCR. Downstream (Sensor 2): After SCR catalyst.
PTT/JPRO required for NOx sensor reset on Volvo/Mack.
DieselVision™ identifies sensor labels and connector pin numbers — upload a close-up photo and the system maps pins automatically.
Connector Pinout
| Pin | Function | Specification |
|---|---|---|
| 1 | CAN High (J1939) | 2.5–3.5V (dominant state) |
| 2 | CAN Low (J1939) | 1.5–2.5V (dominant state) |
| 3 | Power Supply (12V) | 11.5–14.5V |
| 4 | Ground | <0.1V to chassis ground |
Test Procedure
Read NOx sensor 1 and 2 live data at idle
Expected: Sensor 1 (upstream): 200–800 ppm at idle. Sensor 2 (downstream): 10–50% of sensor 1
Tool: OEM diagnostic software
Check NOx sensor power supply voltage
Expected: 11.5–14.5V at power pin
Tool: Multimeter
Check NOx sensor ground circuit
Expected: <0.5 ohms to chassis ground
Tool: Multimeter
Check CAN bus communication to NOx sensor
Expected: No J1939 communication faults for NOx sensor
Tool: OEM diagnostic software
Monitor NOx sensor heater status in live data
Expected: Heater should be active until sensor reaches operating temperature (~300°C)
Tool: OEM diagnostic software
If sensor 1 and sensor 2 read the same value, SCR is not converting
Expected: Downstream NOx should be significantly lower than upstream when SCR is working
Tool: OEM diagnostic software
Common Failure Modes
- •NOx sensor heater failure — sensor does not reach operating temperature
- •NOx sensor internal failure — stuck reading or zero output
- •CAN bus communication fault to NOx sensor
- •NOx sensor contamination from DEF crystallization
- •NOx sensor connector corrosion
- •Upstream NOx sensor reading high — EGR system fault causing high engine-out NOx
Frequently Asked Questions
Where is the upstream NOx sensor located?
The upstream NOx sensor (Sensor 1) is threaded into the exhaust pipe after the DPF and before the SCR catalyst on all major Class 8 platforms.
What is the difference between NOx sensor 1 and NOx sensor 2?
NOx sensor 1 (upstream) measures engine-out NOx before the SCR catalyst. NOx sensor 2 (downstream) measures tailpipe NOx after the SCR. The ECM compares both to calculate SCR conversion efficiency.
Do NOx sensors require a reset after replacement?
Yes — most platforms require a sensor reset procedure using OEM software (Insite, DDDL, DAVIE, PTT) after replacing a NOx sensor.
What fault codes are caused by a failed NOx sensor?
Common fault codes include SPN 2639 (NOx sensor 1), SPN 4364 FMI 18 (SCR efficiency below threshold), and SPN 5246 FMI 31 (DEF inducement).
Snap a photo of your scan tool screen and let Diesel Vision interpret the fault instantly — no manual code lookup required.
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