Peterbilt · SCR NOx Conversion EfficiencyHigh

Peterbilt SPN 4364 FMI 3: SCR NOx Conversion Efficiency Voltage Above Normal

SPN 4364 FMI 3 (FMI 3) — Aftertreatment 1 SCR Catalyst NOx Conversion Efficiency - Voltage Above Normal

4.8/5· 128 reviews

Quick Answer

SPN 4364 FMI 3 on Peterbilt means the SCR NOx conversion efficiency monitor circuit voltage is above the ECM maximum. Test NOx sensor wiring and 5V reference before assuming low SCR efficiency.

Overview: Peterbilt SPN 4364 FMI 3

SPN 4364 tracks how effectively the SCR catalyst converts NOx using upstream and downstream sensor data. FMI 3 specifically flags an electrical voltage-high condition in the efficiency calculation circuit — not confirmed poor SCR performance. ProfessionalDieselRepair.com sees Peterbilt operators confuse FMI 3 with FMI 18 and order unnecessary OneBox assemblies. Work the NOx sensor harness and ACM inputs first.

Repair Snapshot

Time

1 - 4 Hours

Difficulty

Intermediate

Parts Cost

$100 - $900

Dealer Labor

$400 - $850

Tools Required

  • PACCAR Davie4
  • Noregon JPRO
  • Multimeter
  • DEF Refractometer

What SPN 4364 FMI 3 Means

SPN 4364 FMI 3 on Peterbilt trucks with PACCAR MX-11/MX-13 or Cummins aftertreatment means the ECM is receiving a NOx conversion efficiency signal voltage above the calibrated maximum. FMI 3 is an electrical circuit fault on the efficiency monitor input — not a confirmed low SCR efficiency reading (that would be FMI 18). The amber MIL activates and SCR monitor data may be flagged invalid until the circuit is repaired. Common causes include a shorted efficiency sensor signal, damaged NOx sensor harness, or ACM internal reference fault.

Symptoms Drivers Notice

  • Amber check engine or aftertreatment warning lamp
  • NOx conversion efficiency parameter pegged at maximum or showing implausible values on Davie4
  • Possible loss of SCR monitor readiness until fault is cleared
  • Companion NOx sensor voltage codes (SPN 3226 or SPN 3217) may appear with shared harness damage
  • No immediate derate on most calibrations, but inducement may follow if efficiency data stays invalid

Most Likely Root Causes

Downstream NOx sensor signal wire shorted to 5V supply

Very Common

Chafed harness between the OneBox and frame rail can short the NOx sensor signal to reference voltage, driving efficiency voltage above calibrated limits.

Failed outlet NOx sensor with internal short

Common

Thermal cycling and DEF contamination degrade the sensor element, producing a stuck-high voltage output that corrupts efficiency calculations.

ACM internal reference circuit fault

Rare

A failing aftertreatment control module can bias the efficiency input high; confirm only after both NOx sensors and harness test good.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR catalyst for an electrical FMI 3 fault

Why it happens: SPN 4364 names SCR efficiency, so a parts-cannon catalyst swap is ordered without reading the FMI number.

Do this instead: FMI 3 is electrical. Test NOx sensor voltages at the connector. Replace sensors or repair wiring — reserve catalyst replacement for FMI 18 with confirmed poor efficiency after dosing verification.

Replacing both NOx sensors without harness testing

Why it happens: Both sensors are replaced together to save time when only the harness is shorted.

Do this instead: Disconnect both sensors and measure harness-side voltage. A high reading with sensors unplugged confirms a wiring fault, not dual sensor failure.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com reminds Peterbilt techs that SPN 4364 FMI 3 is voltage high — an electrical fault. Do not replace the SCR catalyst for FMI 3. FMI 18 (efficiency low) is the code that points at dosing, DEF, or a dead brick. Pin-test the NOx sensor harness at the OneBox before any parts-cannon catalyst order.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect PACCAR Davie4 or Noregon JPRO and confirm SPN 4364 FMI 3. Record upstream and downstream NOx sensor voltages — typical good sensors output 0.5V-4.5V proportional to NOx ppm on a 5V reference.
  2. Step 2: Key on, engine off: measure 5V reference at the ACM NOx efficiency input connector. Supply should read 4.75V-5.25V. Signal voltage above 4.8V with sensors disconnected indicates harness short to power.
  3. Step 3: Inspect upstream (SPN 3216) and downstream (SPN 3226) NOx sensor connectors for coolant or DEF contamination. Resistance between signal and ground below 1 kΩ at the harness confirms a short.
  4. Step 4: Perform a DEF quality test with a refractometer (32.5% target) and verify DEF tank level — electrical FMI 3 does not require DEF checks, but companion efficiency faults often follow ignored DEF issues.
  5. Step 5: After wiring repair, run a stationary SCR efficiency test in Davie4. Conversion efficiency should calculate 70%-95% at 1800 RPM loaded idle once both NOx sensors report valid data.

Live Data & Freeze-Frame Tips

In Davie4, monitor upstream NOx ppm, downstream NOx ppm, and calculated conversion efficiency. With FMI 3 active, efficiency may read invalid or pegged. After repair, downstream NOx should be 70%-90% lower than upstream at steady cruise with active DEF dosing.

Safety Warnings

  • NOx sensors operate at exhaust temperatures above 300°C — allow the OneBox to cool before connector service.
  • Disconnect batteries before probing ACM harness connectors to prevent module damage.
  • Wear nitrile gloves when handling DEF-contaminated sensor connectors.

When to Limp In vs Tow

SPN 4364 FMI 3 alone does not require towing. Tow if companion inducement codes activate severe derate or if multiple aftertreatment electrical faults appear.

Prevention Tips for Fleets & Owner-Operators

  • Inspect NOx sensor harness routing at every PM — keep looms away from sharp frame edges.
  • Use dielectric grease on NOx sensor connectors after cleaning.
  • Maintain DEF quality above 30% urea concentration to reduce sensor contamination.
  • Address DEF crystallization at the doser before it migrates to NOx sensor bosses.

Related Fault Codes

SPN 4364 FMI 18 — Aftertreatment SCR Catalyst

SCR NOx conversion efficiency low — performance fault vs electrical FMI 3.

SPN 3226 FMI 3 — Outlet NOx Sensor

Outlet NOx sensor voltage high — often the root sensor causing efficiency circuit faults.

SPN 3364 FMI 9 — DEF Quality Sensor

DEF dosing unit communication fault — can cause efficiency problems after electrical repair.

Frequently Asked Questions

What does Peterbilt SPN 4364 FMI 3 mean?

It means the SCR NOx conversion efficiency monitor circuit voltage is above the ECM calibrated maximum. This is an electrical fault, not confirmed low SCR efficiency.

Is FMI 3 the same as low SCR efficiency?

No. FMI 3 is voltage above normal in the circuit. FMI 18 means efficiency is valid but below the required threshold — that is the code associated with catalyst or dosing problems.

Which NOx sensor causes SPN 4364 FMI 3?

Either upstream or downstream sensor can cause it, but the outlet (downstream) sensor is the more common failure point. Test both before replacing.

Can bad DEF cause FMI 3?

Contaminated DEF typically causes FMI 18 (low efficiency), not FMI 3. However, DEF contamination can damage NOx sensors, which then produce voltage-high electrical faults.

What scan tools work on Peterbilt MX-13?

PACCAR Davie4 is the primary OEM tool. Noregon JPRO and Cummins INSITE (on Cummins-powered models) also read SPN 4364 live data.

How do I clear SPN 4364 FMI 3?

Repair the NOx sensor circuit, verify valid efficiency calculations in Davie4, then clear the code. It returns immediately if voltage remains above threshold.

More Peterbilt codes on this system

All Peterbilt fault codes

Guides that go deeper