PACCAR MX-13 NOx Sensor Replacement
The MX-13 uses two NOx sensors — one upstream of the SCR catalyst (inlet) and one downstream (outlet). The ECM compares the two readings to calculate SCR conversion efficiency. A failed NOx sensor causes SCR efficiency faults, DEF inducement, and engine derate. Before replacing a NOx sensor, verify the sensor is actually failed — contaminated DEF, a failed DEF dosing unit, or a plugged DEF injector can cause the same fault codes without a sensor failure. Always verify DEF quality and dosing system function before condemning a NOx sensor.
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Safety — Read Before Starting
- •Allow the exhaust system to cool completely before working near NOx sensors — exhaust temperatures can exceed 1,200°F.
- •Wear heat-resistant gloves when working near the exhaust system.
- •Do not perform this repair immediately after a regeneration event — allow at least 2 hours for the exhaust system to cool.
- •Wear safety glasses throughout the procedure.
- •Do not apply anti-seize to the NOx sensor tip — it will contaminate the sensor element.
Step-by-Step Procedure
Read NOx sensor live data using DAVIE diagnostic software. Compare upstream and downstream NOx readings.
Upstream NOx (before SCR) should be significantly higher than downstream NOx (after SCR) when the SCR is functioning correctly. If both readings are similar, the SCR is not converting NOx — check DEF quality and dosing before replacing sensors.
Verify DEF quality using a refractometer. DEF concentration should be 32.5% ± 1.5%.
Contaminated DEF is the most common cause of SCR efficiency faults. If DEF concentration is out of range, drain and refill the DEF tank before replacing any sensors.
Allow the exhaust system to cool completely before working near the NOx sensors.
NOx sensors are mounted in the exhaust stream. Exhaust temperatures can exceed 1,200°F during operation. Allow at least 2 hours after shutdown.
Identify the failed sensor — upstream sensor is located before the SCR catalyst, downstream sensor is after.
The upstream sensor is typically mounted on the exhaust pipe between the DPF and SCR. The downstream sensor is mounted on the SCR outlet pipe.
Disconnect the NOx sensor electrical connector.
Use a connector pick tool. The NOx sensor connector is a multi-pin connector — do not pry.
Apply penetrating oil to the NOx sensor threads. Allow to soak for 15–30 minutes.
NOx sensors are exposed to high exhaust temperatures and can seize in the bung. Do not force removal without penetrating oil.
Remove the NOx sensor using a NOx sensor socket (22mm or 27mm depending on sensor type).
Turn counterclockwise to remove. If the sensor is seized, apply heat to the bung with a heat gun — do not use an open flame near the exhaust system.
Inspect the sensor bung threads for damage. Clean threads with a thread chaser if necessary.
Damaged bung threads require bung replacement or thread repair before installing the new sensor.
Apply anti-seize to the new NOx sensor threads. Do not apply anti-seize to the sensor tip.
Anti-seize on the sensor tip can contaminate the sensor element and cause inaccurate readings.
Install the new NOx sensor. Torque to specification.
NOx sensor torque is typically 40–50 Nm (30–37 ft-lbs). Verify with OEM service manual for your specific sensor part number.
Reconnect the NOx sensor electrical connector. Verify connector is fully seated and locked.
Route the sensor harness away from hot exhaust components. Use OEM harness clips.
Using DAVIE, clear all fault codes and perform an SCR system test.
DAVIE can perform a forced SCR system test to verify sensor operation before road testing.
Perform a road test at highway speed. Monitor upstream and downstream NOx live data.
After a successful SCR system test, downstream NOx should be significantly lower than upstream NOx. If SCR efficiency faults return, verify DEF dosing rate and DEF injector function.
Frequently Asked Questions
What is the torque spec for PACCAR MX-13 NOx sensors?
MX-13 NOx sensors typically torque to 40–50 Nm (30–37 ft-lbs) with anti-seize applied to the threads. Always verify with the OEM service manual for your specific sensor part number.
What causes SCR efficiency faults on the MX-13?
SCR efficiency faults on the MX-13 are most commonly caused by contaminated DEF, a failed DEF dosing unit, or a plugged DEF injector — not a failed NOx sensor. Always verify DEF quality and dosing system function before replacing NOx sensors.
What fault codes are related to MX-13 NOx sensor issues?
Common related fault codes include SPN 4094 FMI 18 (NOx outlet above normal), SPN 3031 FMI 2 (SCR catalyst efficiency low), SPN 3226 FMI 3 (NOx sensor voltage high), and SPN 3226 FMI 4 (NOx sensor voltage low).
How long does MX-13 NOx sensor replacement take?
MX-13 NOx sensor replacement typically takes 45–90 minutes, including DEF quality verification and post-replacement SCR system testing.
Do I need DAVIE software to replace the MX-13 NOx sensor?
DAVIE is required to perform the SCR system test after replacement to verify the new sensor is functioning correctly. A standard OBD-II scanner can read fault codes but cannot perform the SCR system test.