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PACCAR SPN 3364 FMI 9: DEF Quality Abnormal Update

SPN 3364 FMI 9 (FMI 9) — Aftertreatment DEF Quality - Abnormal Update Rate

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Quick Answer

SPN 3364 FMI 9 means PACCAR cannot verify DEF quality due to sensor communication failure or contaminated fluid. Test DEF concentration and inspect the tank module harness.

Overview: PACCAR SPN 3364 FMI 9

DEF quality sensors confirm urea concentration for SCR dosing. When the ECM stops receiving valid quality updates, PACCAR sets SPN 3364 FMI 9 and may limit engine performance.

Repair Snapshot

Time

1 - 4 Hours

Difficulty

Intermediate

Parts Cost

$100 - $600

Dealer Labor

$400+

Tools Required

  • PACCAR Davie4
  • Jaltest
  • Multimeter

What SPN 3364 FMI 9 Means

SPN 3364 FMI 9 on PACCAR means the DEF quality sensor stopped providing valid update messages. The ECM cannot verify 32.5% urea concentration. Inspect module wiring, DEF contamination, and sensor operation before replacing the quality sensor or tank module.

Symptoms Drivers Notice

  • DEF quality warning on dash display
  • Amber MIL with emissions system message
  • Quality reading frozen or absent in scan tool
  • SCR efficiency codes may follow if unresolved
  • DEF appears discolored or has sediment at tank bottom

Most Likely Root Causes

Contaminated or diluted DEF in tank

Common

Water, diesel fuel, or old DEF breaks concentration below 32.5% and triggers quality faults.

Failed DEF quality sensor or module

Common

Ultrasonic or conductivity sensor failure stops valid update messages.

Corroded tank module harness connector

Occasional

DEF vapor attacks pins and causes intermittent communication loss.

Common Misdiagnoses (Don't Waste Parts)

Replacing quality sensor without draining contaminated DEF

Why it happens: Sensor is blamed when fluid is the problem.

Do this instead: Parts-cannon warning — refractometer-test and flush tank if DEF is below 30% urea before sensor replacement.

Adding fresh DEF on top of contaminated fluid

Why it happens: Top-off is faster than drain and flush.

Do this instead: Drain tank completely, flush with distilled water, refill with certified DEF.

Master Tech Commentary

Mechanic's Notes

On PACCAR, always refractometer-test DEF before replacing the quality sensor. Contaminated fluid in the tank will set the fault again immediately on new sensor install.

Step-by-Step Diagnostic Procedure

  1. Step 1: Check DEF quality reading in PACCAR Davie4 — valid ISO DEF should report near 32.5% urea concentration.
  2. Step 2: Sample DEF from tank bottom and test with refractometer — readings below 30% indicate contamination or dilution.
  3. Step 3: Inspect DEF tank module connector for corrosion on quality sensor pins.
  4. Step 4: Drain and flush DEF tank if diesel fuel, coolant, or water contamination is confirmed.
  5. Step 5: Verify quality sensor harness continuity — resistance should change smoothly when DEF temperature shifts.

Live Data & Freeze-Frame Tips

Monitor DEF quality % and tank temp in PACCAR Davie4. Quality should read 30–35% for valid ISO fluid. Frozen or zero reading with good fluid means sensor circuit fault.

Safety Warnings

  • Contaminated DEF is hazardous waste — dispose per local regulations.
  • Wear chemical-resistant gloves when draining DEF tanks.

When to Limp In vs Tow

Tow if SCR inducement derate is active and DEF quality cannot be verified before extended driving.

Prevention Tips for Fleets & Owner-Operators

  • Use only API-certified DEF from sealed containers.
  • Never put diesel fuel, water, or additives in the DEF tank.
  • Replace DEF if truck sits unused more than 12 months.

Related Fault Codes

SPN 4364 FMI 18 — SCR NOx Efficiency

SCR efficiency faults follow unresolved DEF quality issues.

SPN 3361 FMI 9 — DEF Tank Level

Tank module communication faults on shared harness.

Frequently Asked Questions

Can bad DEF cause SPN 3364 FMI 9?

Yes — contaminated DEF can trigger quality faults even with a good sensor.

How do I test DEF quality?

Use a refractometer — target 32.5% urea for ISO 22241 DEF.

Do I need to drain the DEF tank?

Yes, if contamination is confirmed — flushing prevents immediate repeat faults.

Will this code cause derate?

It can progress to SCR inducement if DEF quality stays unverified.

More PACCAR codes on this system

All PACCAR fault codes

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