Catalyst System Efficiency Below Threshold (DOC)
The HINO-P0420 (Hino) diesel fault code means: Catalyst System Efficiency Below Threshold (DOC). This is a serious severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- intermediate
- Estimated cost
- $500-$3,500
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Common Symptoms
- DPF / Regen lamp illuminated
- Progressive derate (5 mph in final stage)
- Frequent request for manual regen
- Reduced power and fuel economy
- Elevated EGTs during regen
Probable Causes (Ranked by Likelihood)
- Short-haul urban delivery duty cycle preventing successful regens (Hino 195 box truck especially) Very Likely
- DPF ash load exceeding service threshold (Hino DPF cleaning interval 200K-300K mi) Likely
- Upstream fault (EGR, injector, rail pressure) preventing proper regen temperature Likely
- Failed EGT or differential pressure sensor Possible
- Plugged delta-P sample tubes giving invalid readings Possible
Frequently Asked Questions
What does Hino code P0420 mean?
Hino diagnostic trouble code P0420 (Catalyst System Efficiency Below Threshold (DOC)) is a serious-severity fault on Hino medium-duty diesel engines. It indicates a problem in the DPF / aftertreatment that should be verified with a scan tool (Hino DX3 / DX4) before replacing any parts.
Can I drive my Hino truck with P0420?
Short runs may be possible but do not ignore — Hino medium-duty aftertreatment faults cascade quickly into 5-mph derate or no-restart. Schedule diagnosis immediately.
How much does it cost to fix Hino P0420?
Estimated repair cost runs $500-$3,500. Sensor, wiring, and DEF-related repairs fall at the low end; DPF service, turbo, injector, and EGR cooler work fall at the high end and may require dealer-level programming.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
Confirm P0420 on Hino DX3. Capture soot %, ash %, and distance/time since last successful regen.
Attempt manual / parked regen — outlet EGT should exceed 1000°F for 20+ minutes. Failure to climb indicates upstream DOC / HC doser issue.
Remove and clean delta-P sample lines — soot plugs are a recurring cause of bogus P244A/B codes on Hino.
If ash load is over limit, send DPF for bake-and-blow service or replace; ash cannot be regenerated away.
After repair, reset soot counters with DX3 and drive a representative duty cycle to verify regen completes.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- SAE J1939-73Application Layer: Diagnostics (defines the SPN/FMI fault-identifier framework)
- SAE J2012Diagnostic Trouble Code Definitions (defines P0420 as a generic OBD-II code)
P0420 on Other Platforms
The same code ID appears across other engines, vehicles, and equipment. Diagnostic flow varies by platform — see the matching breakdown:
Commonly Affected Vehicles
- Hino 155 / 155h with J05 5.1L
- Hino 195 / 195h (hybrid) with J05D 5.1L
- Hino 258 / 258LP / 268 with J08 7.7L
- Hino 338 / 338CT with J08 7.7L
- Hino 358 / 358CT (post-2019 J-series / A09C)
- Toyota Dyna / Hino-branded export chassis with J05/J08
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