MID 128 PID 409 FMI 0 Volvo/Mack: Meaning, Causes & Fix

What MID 128 PID 409 FMI 0 means

MID 128 PID 409 FMI 0: SCR NOx conversion efficiency below threshold (engine ECU).

Applies to: Volvo D11/D13/D16, Mack MP7/MP8, EPA13 and later.

Severity: High — derate chain.

Symptoms you'll see

MIL + SCR telltale
PTT shows native MID/PID; dash shows generic SCR warning

Likely causes (ranked by probability)

DEF doser under-dosing (~35%)
NOx sensor drift (~25%)
SCR catalyst degradation (~25%)
Exhaust leak upstream of SCR (~15%)

Diagnose, don't guess. Aftertreatment parts are expensive and the ranked list above is where they get wasted — follow the steps below before replacing anything.

Diagnostic steps

Run PTT's guided SCR efficiency test — doser, sensors, catalyst in sequence.
Manual path: dosing quantity test → sensor plausibility → catalyst assessment.
Inspect exhaust clamps and flex joints upstream of the catalyst.
Repair, verify, reset inducement if armed.

Tools that read, clear and verify this code

PTT + Vocom II for the guided sequence; CV scanners read/clear the J1939 translation. For the full per-platform picture see the Volvo D13 fault codes page and our Aftertreatment fault codes.

Frequently Asked Questions

Can I drive with MID 128 PID 409 FMI 0 active?

Severity on this code: High — derate chain. If a derate countdown is armed, use the remaining runtime to reach a shop — not to finish the week. Verify the root cause with the diagnostic steps above rather than clearing and hoping.

Will clearing MID 128 PID 409 FMI 0 without repairs work?

The code will return — usually within one drive cycle — and on aftertreatment-related faults the inducement countdown often shortens each time. Repair first, verify with the guided test, then clear.

Which scanner reads MID 128 PID 409 FMI 0?

PTT + Vocom II for the guided sequence; CV scanners read/clear the J1939 translation.

MID 128 PID 409 FMI 0 — Volvo/Mack