2015 Peterbilt 579 Paccar MX-13 - Intermittent Derate - SPN 5246 FMI 0

Question

Truck is stuck in a 5 MPH derate and it's killing my schedule. It started with a 'Check Engine' light and then moved to 'Stop Engine' within 50 miles. I'm getting codes for NOx conversion efficiency being below threshold.

I've checked the DEF tank and it's full of good 32.5% fluid (tested with a refractometer). The pump seems to be priming because I can hear it, and I'm seeing pressure on the DAVIE tool, but the downstream NOx sensor just isn't seeing the drop in numbers it wants to see.

CoolantKai · Answer

✅ Confirmed fix: Found the SCR catalyst was actually poisoned due to an internal coolant leak from the EGR cooler. The coolant was coating the SCR brick, preventing the chemical reaction with the DEF. Replaced the EGR cooler and the entire SCR drum. After the SCR replacement and a long test drive, the NOx conversion returned to 95% efficiency and the derate cleared.

HighwayHelen · Answer

Check the DEF decomposition pipe. These MX-13s are notorious for building up 'crystal' towers right where the injector sprays into the pipe. If there is a massive chunk of dried DEF there, the spray won't atomize and you'll get poor conversion codes even with a brand new nozzle.

2015 Peterbilt 579 Paccar MX-13 - Intermittent Derate - SPN 5246 FMI 0

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