NOx sensors play a critical role in your truck’s emissions system. If you’re getting codes like SPN 4094, SPN 3490, or seeing frequent regens and poor fuel economy, it could point to a failing NOx sensor.

Here’s how to test, diagnose, and confirm if your NOx sensor is bad—using a diesel laptop scanner.

🛠 What You’ll Need:

• Diesel laptop diagnostic kit
  

• Your truck parked with the ignition on (engine off or idling)

• Access to live data and bi-directional test functions

🔍 Step-by-Step: Test a NOx Sensor

1. Plug in your adapter and open your diagnostic software

2. Connect to the ECM and navigate to Live Data > Emissions / Aftertreatment

3. Locate the readings for:

   • Inlet NOx sensor (before SCR)

   • Outlet NOx sensor (after SCR)

4. At idle or while driving, compare values—large differences are normal, but flat or 0 values = bad sensor

5. Perform a NOx Sensor Function Test (if available in your software)

6. If the sensor fails the test, it likely needs to be replaced

⚠️ Watch for These Symptoms:

• Frequent DPF regens

• SPN 4094, 4364, 3490 fault codes

• Poor DEF dosing performance

• Derate or check engine light staying on

💡 Pro Tip:

After replacing a NOx sensor, always clear fault codes and run a regen to reset the system. Some ECMs also require a NOx sensor relearn or calibration step.

👉 Need a scanner that reads and tests NOx sensors on all major diesel engines? Click here to shop diesel laptop kits