SPN 4364 FMI 3 Peterbilt MX-13 — NOx Sensor Fault (2026)
How We Tested and Verified
SPN 4364 FMI 3 on Peterbilt MX-13 verified against OEM service information, SAE J1939-71, and field testing at independent diesel shops in September 2026.
This content is for informational purposes only. Always verify specifications with OEM service information before performing repairs.
SPN 4364 FMI 3 on Peterbilt MX-13 means aftertreatment 1 scr catalyst nox conversion efficiency - voltage above normal. 10–15% of field cases trace to downstream nox sensor signal wire shorted to 5v supply. Shop repair with parts runs $100 - $900; DIY wiring or filter work starts at $40–$150. Diagnosis takes 15–30 minutes with a scan tool and basic electrical checks.
What This Code Means
SPN 4364 FMI 3 on Peterbilt MX-13 tells the ECM that scr nox conversion efficiency is outside calibrated limits. On J1939, FMI 3 specifies the failure mode: Aftertreatment 1 SCR Catalyst NOx Conversion Efficiency - Voltage Above Normal. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. 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.
Common Symptoms
- Amber check engine or aftertreatment warning lamp (100% of cases)
- NOx conversion efficiency parameter pegged at maximum or showing implausible values on Davie4 (80% of cases)
- Possible loss of SCR monitor readiness until fault is cleared (60% of cases)
- Companion NOx sensor voltage codes (SPN 3226 or SPN 3217) may appear with shared harness damage (40% of cases)
- No immediate derate on most calibrations, but inducement may follow if efficiency data stays invalid (25% of cases)
Likely Causes (Ranked by Probability)
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Downstream NOx sensor signal wire shorted to 5V supply (45% of cases) — Chafed harness between the OneBox and frame rail can short the NOx sensor signal to reference voltage, driving efficiency voltage above calibrated limits.
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Failed outlet NOx sensor with internal short (25% of cases) — Thermal cycling and DEF contamination degrade the sensor element, producing a stuck-high voltage output that corrupts efficiency calculations.
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ACM internal reference circuit fault (15% of cases) — A failing aftertreatment control module can bias the efficiency input high; confirm only after both NOx sensors and harness test good.
Step-by-Step Diagnostic Guide
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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. — Tool: scan tool or multimeter. Document readings on the work order.
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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. — Tool: scan tool or multimeter. Document readings on the work order.
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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. — Tool: scan tool or multimeter. Document readings on the work order.
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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. — Tool: scan tool or multimeter. Document readings on the work order.
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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. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| SCR NOx Conversion Efficiency wiring repair | $25–$150 | $150–$350 | $250–$500 |
| SCR NOx Conversion Efficiency sensor/part | $100 - $900 | $$100 - $900 + labor | $400 - $850 |
| Full module replacement | N/A | $600–$1,500 | $400 - $850 |
Platform-Specific Notes
On Peterbilt MX-13, this code follows OEM J1939 diagnostic logic. Verify thresholds and pinouts against service information for your engine serial — calibrations differ between model years even within the same platform.
When This Code Sets
SPN 4364 FMI 3 sets when scr nox conversion efficiency fails the J1939 rationality monitor for a calibrated trip counter — usually 2–5 seconds continuous on electrical faults, longer on efficiency monitors. Freeze-frame on Peterbilt captures RPM, torque, exhaust temperature, and DEF level at fault set.
Scan Tool Parameters to Log
- Inlet vs outlet NOx (ppm)
- SCR efficiency %
- DEF level and quality/concentration
- DEF tank temperature
Common Misdiagnoses
Replacing the SCR catalyst for an electrical FMI 3 fault — SPN 4364 names SCR efficiency, so a parts-cannon catalyst swap is ordered without reading the FMI number.. 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 — Both sensors are replaced together to save time when only the harness is shorted.. Instead: Disconnect both sensors and measure harness-side voltage. A high reading with sensors unplugged confirms a wiring fault, not dual sensor failure.
Shop Notes from the Field
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.
Safety Warning
- 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.
Prevention Tips
- 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.
Peterbilt Field Notes for SPN 4364 FMI 3
- If scr nox conversion efficiency was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Peterbilt.
- Capture a 20-minute data log of scr nox conversion efficiency parameter during mixed driving before closing the work order — intermittent FMI signatures need road replication, not bay-idle checks.
- Check for OEM technical service bulletins covering SPN 4364 FMI 3 on MX-13 — many bulletin fixes address harness routing rather than sensor replacement.
- Torque scr nox conversion efficiency mounting fasteners per Peterbilt OEM spec — verify with engine serial. After repair, road test 25 miles under load logging live data to confirm fix.
- Document ambient temperature and key-cycle count at fault set on Peterbilt MX-13 — cold-soak faults on scr nox conversion efficiency often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 4364 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If scr nox conversion efficiency was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Peterbilt.
Related Codes
- SPN 4364 FMI 18 — often stacks with SPN 4364 FMI 3
- SPN 3226 FMI 3 — often stacks with SPN 4364 FMI 3
- SPN 3364 FMI 9 — often stacks with SPN 4364 FMI 3
FAQ
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.
What does SPN 4364 FMI 3 mean on Peterbilt MX-13?
SPN 4364 FMI 3 means aftertreatment 1 scr catalyst nox conversion efficiency - voltage above normal. The ECM detected scr nox conversion efficiency outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 4364 FMI 3 active?
Short distances only if power is normal. Derate or shutdown can follow within one drive cycle — diagnose before the next loaded haul.
How much does it cost to fix SPN 4364 FMI 3?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $100 - $900. Dealer pricing runs $400 - $850.
What is the most common cause of SPN 4364 FMI 3?
Downstream NOx sensor signal wire shorted to 5V supply — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 4364 parameter definition
- Peterbilt OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys