How Do You Test a NOx Sensor?

A faulty NOx sensor cannot be diagnosed only by the check engine light.
A proper diagnosis requires fault code scanning, live data analysis and inspection of the SCR system.

Modern diesel vehicles store NOx-related errors directly in the ECU.

A NOx sensor is tested using an OBD diagnostic scanner. Fault codes such as P2200 or incorrect live sensor readings usually indicate a problem in the SCR or diesel emission system.

Common Symptoms Before Testing

Before performing a diagnostic check, drivers often notice symptoms such as:

• check engine light
• “emission system fault” warning
• increased AdBlue consumption
• reduced engine performance
• limp mode activation

These signs usually indicate a problem in the SCR or diesel emission system.

OBD Diagnosis – Reading Fault Codes

The most effective way to test a NOx sensor is by using an OBD diagnostic scanner.

Common fault codes include:

• P2200 → NOx Sensor Circuit
• P2201 → Sensor Performance Problem
• P2202 → Low Sensor Voltage
• P2203 → High Sensor Voltage

These codes help identify whether the problem comes from the sensor itself or from the electrical system.

Checking Live Sensor Data

Besides fault codes, live data analysis is extremely important.

Technicians usually check:

• NOx values before and after SCR
• sensor response speed
• exhaust gas temperature
• AdBlue injection values

Incorrect or unstable readings often indicate a defective sensor.

Inspecting Wiring and Connectors

Not every NOx-related fault means the sensor is damaged.

The following should also be inspected:

• damaged wiring
• loose connectors
• corrosion
• moisture inside plugs

Electrical issues can create symptoms similar to a failed sensor.

When Should the Sensor Be Replaced?

Replacement is usually necessary if:

• fault codes return repeatedly
• live data remains incorrect
• the vehicle enters limp mode
• emissions stay outside normal range

Can You Drive with a Faulty NOx Sensor?

Short-term driving may still be possible, but it is not recommended.

A faulty sensor can cause:

• higher AdBlue consumption
• SCR system damage
• increased emissions
• poor engine performance

Next Article

➡ NOx Sensor Cleaning vs Replacement – Which Option Is Better?