2014 Hino 268 J08E - P0420 Code and Exhaust Brake Not Working

Question

Working on a Hino 268 box truck. The P0420 code is active for Catalyst System Efficiency. Also noticed that the exhaust brake is no longer engaging when the driver lifts off the throttle. The driver says he noticed a drop in fuel economy lately too.

I've checked the vacuum lines to the exhaust brake actuator and the solenoid seems to be clicking when triggered via the scan tool. However, the flap doesn't seem to be moving. I'm wondering if the two issues are related or if I just have a bad cat and a coincidental mechanical failure of the brake.

DieselDan_77 · Accepted Answer

✅ Confirmed fix: The exhaust brake butterfly valve was seized in the partially closed position due to carbon buildup. This was creating high backpressure and disrupting the exhaust flow readings, which tripped the P0420 catalyst efficiency code. I replaced the exhaust brake housing assembly and cleared the codes; both the brake function and the efficiency code have stayed corrected.

RustBeltRandy · Answer

Hinos are notorious for the Burner system issues. Check the atomization air compressor and the burner nozzle. If the burner isn't getting the catalyst hot enough during the active cycles, you'll get that efficiency code. The exhaust brake might just be a frozen pivot pin, try some PB Blaster on the linkage.

2014 Hino 268 J08E - P0420 Code and Exhaust Brake Not Working

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