2018 Peterbilt 579 - Derate with SPN 3216 FMI 20 - NOx Sensor Drift

Question

Running over the Grapevine yesterday and I got a solid MIL followed 10 minutes later by a 5 mph derate warning. Davco looks clean and I just did my fuel filters 5k miles ago. I'm seeing SPN 3216 FMI 20 on the dash display. The truck feels like it's running fine but I can't risk the derate in the middle of this haul.

I tried clearing the codes with my basic scanner but the code goes active again as soon as the exhaust hits operating temp. I checked the harness at the inlet NOx sensor and it looks okay, no visible rubbing or green crusties in the connector.

MisfireMatt · Answer

You need to perform a stationary regen and watch the NOx conversion efficiency on a laptop. If your inlet NOx reading is higher than your outlet but the delta is too small, it will pop that 3216 code. Also check for exhaust leaks before the SCR.

BackyardBilly · Answer

✅ Confirmed fix: The PACCAR MX-13 is notorious for failing inlet NOx sensors with that specific FMI 20 drift code. The internal ceramic element gets soot-fouled or cracked. Replaced the inlet NOx sensor (located on the turbo discharge pipe) and performed a 'Reset Learned SCR Data' via Davie4. The code stayed inactive and the derate cleared immediately.

2018 Peterbilt 579 - Derate with SPN 3216 FMI 20 - NOx Sensor Drift

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