SCR NOx Sensor Circuit High
The SCR NOx sensor voltage is higher than expected.
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What it means (plain English)
Your truck's SCR NOx sensor is reporting a voltage that's higher than normal, indicating possible electrical issues or sensor faults.
What the computer is actually seeing
The Engine Control Module (ECM) sees a voltage signal from the NOx sensor that's above the normal operating range, typically above 3.0 volts.
What a healthy reading looks like
Normal NOx sensor output should be between 0.5 and 2.5 volts under typical operating conditions.
Guided diagnostic — the DiagCoach way
Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.
- 11. Check the NOx sensor wiring for damage, fraying, or disconnections.
- 22. Inspect the connectors for corrosion or poor contact and clean or repair as necessary.
- 33. Measure the voltage at the NOx sensor connector. It should be between 0.5 and 2.5 volts when idling.
- 44. If the voltage is above 3.0 volts, check for a short circuit to power or a short in the sensor itself.
- 55. If wiring and connections are good, replace the NOx sensor and retest.
- 66. Clear the fault code and see if it returns after a test drive.
Common causes
- Damaged NOx sensor
- Wiring issues or open circuit
- Corroded connectors
- Faulty ECM or software issues
- Contaminated exhaust or sensor poisoning
Typical repair cost
$$$
Related codes
Frequently asked questions
Is it safe to drive with SPN 0428 active?
Moderately safe, but fix it soon as it affects emissions.
Will this code come back after clearing it?
Yes, if the issue isn't fixed, it will likely come back.
Why does this code appear with SPN 0429?
They both relate to NOx sensor performance; one could be failing due to the other's fault.
Working a real vehicle right now?
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