What is SPN 1235 & How to Solve It?

What is SPN 1235:

SPN 1235 refers to the Aftertreatment 1 Intake NOx Sensor. When this code appears without a specific FMI, it typically signals a general issue with the NOx sensor — such as data dropout, inconsistent readings, or communication problems.

This sensor measures nitrogen oxide (NOx) levels before they enter the SCR system. If it fails, the engine may incorrectly dose DEF or skip regeneration entirely.

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Why This Happens

• NOx sensor failing or drifting.
• Damaged sensor wiring or connector.
• Moisture intrusion in the sensor.
• Slow or erratic data updates.
• Internal sensor heating element failure.

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How to Fix It

1. Check for additional fault codes to identify exact FMI details.
2. Inspect the NOx sensor for signs of damage or contamination.
3. Test sensor wiring for continuity and corrosion.
4. Replace the NOx sensor if values are unstable or data is missing.
5. Clear the code and verify real-time readings during operation.

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How to Avoid It Before It Happens

1. Replace NOx sensors at intervals recommended by the manufacturer.
2. Keep connectors clean and sealed.
3. Avoid DEF or coolant spills near the sensor.
4. Run emissions diagnostics regularly to catch early signal issues.