PACCAR Fault Code P3973: NOx Sensor After Catalyst – O₂ Concentration, Reacting Too Slow

Code details

Manufacturer

PACCAR (Kenworth & Peterbilt)

Applies to

PACCAR MX-13 (2017–2020)

Fault code

P3973

J1939 SPN

4382

FMI

11 — Root cause not known

Truck models in source

Kenworth T680, T880, W900, T800; Peterbilt 579, 389, 567, 367

What to do next

• Read active and inactive codes with the OEM service tool and note the occurrence count.
• FMI 11 tells the shop the type of failure to test for: root cause not known.
• Have a heavy-duty diesel technician confirm the fault against the engine or chassis service manual before replacing parts.

Related fault codes

• PACCAR Fault Code P3973 — Heater Element NH₃ Sensor – Heating Failed During Warm-Up
• PACCAR Fault Code P3971 — NOx Sensor After Catalyst – O₂ Concentration, Too High
• PACCAR Fault Code P3972 — NOx Sensor After Catalyst – O₂ Concentration, Too Low
• PACCAR Fault Code P3971 — Heater Element NH₃ Sensor – Failed to Maintain the Operating Temperature
• PACCAR Fault Code P3972 — Heater Element NH₃ Sensor – Failed to Maintain the Operating Temperature