How to Tell If Your NOx Sensor Is Failing (And What to Do About It)

How to Tell If Your NOx Sensor Is Failing (And What to Do About It)

 

How to Tell If Your NOx Sensor Is Failing (And What to Do About It)

Modern diesel engines rely heavily on NOx sensors to control emissions and optimize engine performance. When a NOx sensor starts to fail, it doesn’t just impact emissions — it can trigger engine derates, cause costly downtime, and lead to failed inspections.

If your truck or heavy equipment is throwing fault codes or running rough, a bad NOx sensor could be the cause.

In this post, we’ll explain:

  • What NOx sensors do

  • Symptoms of a failing NOx sensor

  • Common fault codes to watch for

  • How to fix or replace them quickly

🧪 What Is a NOx Sensor?

A NOx (Nitrogen Oxide) sensor measures the concentration of nitrogen oxides in the exhaust system. Diesel engines typically use two sensors:

  • Upstream NOx Sensor (before the SCR catalyst)

  • Downstream NOx Sensor (after the SCR catalyst)

Together, they help your engine's control module determine how efficiently the SCR system is working and adjust fuel injection and DEF dosing.

⚠️ Common Symptoms of a Failing NOx Sensor

If your NOx sensor is on the way out, here are the most common signs:

🚨 1. Check Engine Light or Fault Codes

  • Fault codes like SPN 4094, SPN 3226, or P2201 are usually the first clue.

  • These indicate issues with sensor voltage, heater performance, or signal mismatch.

🐢 2. Loss of Engine Power (Derate Mode)

The ECM may force the engine into derate to comply with emissions regulations.

⛽ 3. Poor Fuel Economy

A failed sensor can result in incorrect fuel/air mixture and DEF dosing, leading to increased fuel consumption.

📉 4. Failed Emissions Test or Regeneration Issues

If NOx readings are outside expected limits, you may not pass inspection — or the DPF system may fail to regen properly.

🔍 Most Common NOx Sensor Fault Codes

Fault Code Meaning
SPN 4094 / FMI 18 NOx sensor signal out of range
SPN 3226 / FMI 10 NOx sensor heater circuit fault
P2201 Sensor circuit range/performance issue
SPN 4096 Downstream sensor reading mismatch

These codes typically indicate the sensor is reading incorrectly or not heating up to operating temperature.

🛠️ What to Do If You Suspect a Bad NOx Sensor

✅ 1. Confirm with Diagnostics

  • Use software like Cummins Insite, Detroit DiagnosticLink, or TEXA

  • Check live NOx readings and sensor temperatures

✅ 2. Inspect Sensor and Wiring

  • Look for corrosion, soot buildup, or connector damage

  • Check for melted harnesses near exhaust components

✅ 3. Replace the Sensor

  • Always match the OEM part number

  • Replace upstream and downstream sensors if fault codes are unclear

  • Clear codes after installation and test under load

🛒 Where to Buy Quality NOx Sensors

At AftertreatmentParts.com, we stock OEM-quality NOx sensors compatible with:

  • Cummins ISX/X15

  • PACCAR MX13

  • Volvo D13

  • Detroit DD13/DD15

  • International/MaxxForce

✅ In-stock and ready to ship across Canada and the U.S.
✅ 365-day warranty on most sensors
✅ Expert support by phone or email

👉 Shop NOx Sensors Now

🧭 Final Thoughts

A failed NOx sensor is more than a nuisance — it can cost you time, fuel, and productivity. Stay ahead of emissions issues by replacing bad sensors early and avoiding forced derates.

If you’re unsure which sensor you need, contact our team — we’ll help you find the right part and get it shipped fast.

 

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