2018 Peterbilt 579 Paccar MX-13 SPN 4364 FMI 18 - SCR Efficiency Low

Question

I'm fighting a persistent SCR efficiency code on my 579. It started a few days ago after a long haul through the mountains. I've got the SPN 4364 FMI 18 popping up on the dash and the DEF light is flashing. I'm about 100 miles away from an engine derate and I really need to clear this up before I'm stuck crawling to a dealer.

I checked the DEF tank and the fluid looks clear, not cloudy or anything. I also pulled the dosing valve and it didn't look overly carboned up, but I gave it a quick clean anyway. Ran a forced regen and it completed, but the code came back within 30 minutes of hitting the highway. Nitrogen oxide sensors are reading, but I'm not sure if the values are accurate.

ScannerSandy · Answer

You need to check the DEF spray pattern. Just because the nozzle looks clean doesn't mean it's atomizing correctly. Pull the injector, put it in a bucket, and run the dose test via DAVIE software. If you see a stream instead of a fine mist, that's your problem. Also, check for 'crystallization' inside the decomposition pipe where the doser mounts.

OwnerOpOlly · Answer

✅ Confirmed fix: The issue was a drifted inlet NOx sensor. On these MX-13 engines, if the inlet sensor reads lower than actual, the ECU doesn't command enough DEF, triggering the 4364 efficiency code. I swapped the inlet NOx sensor (Part #22790-5301R) and performed the 'SCR Crystal Sublimation' drive cycle. The efficiency monitors passed and the derate cleared immediately.

2018 Peterbilt 579 Paccar MX-13 SPN 4364 FMI 18 - SCR Efficiency Low

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