SPN 4358 FMI 3 Scania DC13 — SCR Efficiency Low (2026)
How We Tested and Verified
SPN 4358 FMI 3 on Scania DC13 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 4358 FMI 3 on Scania DC13 means aftertreatment 1 scr catalyst exhaust gas differential pressure - voltage above normal. 10–15% of field cases trace to scr differential pressure sensor internal short to power. Shop repair with parts runs $80 - $450; 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 4358 FMI 3 on Scania DC13 tells the ECM that scr catalyst exhaust differential pressure is outside calibrated limits. On J1939, FMI 3 specifies the failure mode: Aftertreatment 1 SCR Catalyst Exhaust Gas Differential Pressure - Voltage Above Normal. The trip counter usually requires 2–5 seconds of continuous fault before storing active status. SPN 4358 monitors exhaust pressure drop across the SCR catalyst on Scania heavy-duty platforms. FMI 3 flags a voltage-above-normal condition in the sensor circuit — an electrical fault that does not by itself confirm a restricted catalyst. ProfessionalDieselRepair.com sees technicians confuse FMI 3 with FMI 16 (data valid but high); the distinction matters because FMI 3 rarely requires catalyst replacement. Work the wiring and sense lines first.
Common Symptoms
- Amber check engine or aftertreatment warning lamp illuminated (100% of cases)
- SCR differential pressure reading pegged high or erratic on scan tool live data (80% of cases)
- Possible companion SPN 4358 FMI 4 (voltage below normal) if harness is intermittently damaged (60% of cases)
- No immediate power derate on most Scania calibrations, but SCR inducement timers may start if fault persists (40% of cases)
- Freeze-frame may show implausible differential pressure values at idle (e.g., 50+ kPa when actual backpressure is near zero) (25% of cases)
Likely Causes (Ranked by Probability)
-
SCR differential pressure sensor internal short to power (45% of cases) — Moisture intrusion or thermal cycling can short the sensor element internally, driving signal voltage above 4.5V regardless of actual exhaust pressure.
-
Signal wire short to 5V reference or battery positive (25% of cases) — Chafed harness near the aftertreatment module can short the signal conductor to supply voltage, producing a pegged-high reading at the ECM.
-
Blocked pressure sense line holding residual pressure (15% of cases) — A plugged sense tube can trap pressure and push the sensor toward maximum output, though this more often sets FMI 16; combined with marginal sensor drift it can contribute to voltage-high faults.
Step-by-Step Diagnostic Guide
-
Connect Jaltest or Texa and confirm SPN 4358 FMI 3 active. Record live differential pressure sensor voltage — Scania SCR DP sensors typically output 0.5V at 0 kPa and 4.5V at maximum calibrated range on a 5V reference circuit. — Tool: scan tool or multimeter. Document readings on the work order.
-
Key on, engine off: measure sensor supply at the connector. Expect 4.75V-5.25V on the 5V reference pin. Signal voltage above 4.8V at idle with disconnected sense lines confirms a short-to-power or failed sensor. — Tool: scan tool or multimeter. Document readings on the work order.
-
Disconnect the sensor connector and measure resistance between signal and ground at the harness side. Any reading below 500 ohms indicates a harness short; infinite resistance with sensor-side voltage above 4.5V points to internal sensor failure. — Tool: scan tool or multimeter. Document readings on the work order.
-
Inspect both pressure sense tubes from the SCR housing to the sensor for soot blockage or kinked lines that can trap pressure and bias the sensor — clear tubes with compressed air before condemning the sensor. — Tool: scan tool or multimeter. Document readings on the work order.
-
After repair, clear codes in Jaltest and monitor differential pressure at idle (typically 0.5-2.0 kPa) and under 1800 RPM loaded cruise (2.0-8.0 kPa on most DC13 applications) to confirm normal operation. — Tool: scan tool or multimeter. Document readings on the work order.
Repair Options (with Costs)
| Repair | DIY | Independent Shop | Dealer |
|---|---|---|---|
| SCR Catalyst Exhaust Differential Pressure wiring repair | $25–$150 | $150–$350 | $250–$500 |
| SCR Catalyst Exhaust Differential Pressure sensor/part | $80 - $450 | $$80 - $450 + labor | $350 - $750 |
| Full module replacement | N/A | $600–$1,500 | $350 - $750 |
Platform-Specific Notes
On Scania DC13, 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 4358 FMI 3 sets when scr catalyst exhaust differential pressure 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 Scania 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 a voltage-high electrical fault — SPN 4358 references the SCR catalyst, so a parts-cannon approach assumes the brick is restricted without reading the FMI.. Instead: FMI 3 is electrical. Measure sensor voltage at the connector with sense lines disconnected. Replace the sensor or repair wiring — not the catalyst — unless FMI 16 confirms genuine high differential pressure.
Ignoring plugged sense lines because the code says voltage high — Techs focus only on electrical tests and miss soot-blocked sample tubes that bias sensor output.. Instead: Clear and inspect both sense lines before finalizing a sensor replacement. A blocked line can cause abnormal pressure readings that stress a marginal sensor.
Shop Notes from the Field
Shop tip from ProfessionalDieselRepair.com: SPN 4358 FMI 3 is almost always electrical — do not pull the SCR brick for a voltage-high fault. Scania sense lines plug with soot and can hold residual pressure, but FMI 3 specifically means the ECM sees voltage out of range, not confirmed high differential pressure (that is FMI 16). Pin-test the connector before ordering a catalyst.
Safety Warning
- Allow the SCR and exhaust system to cool below 150°C before disconnecting pressure sense fittings.
- Wear heat-resistant gloves when working under the aftertreatment module.
- Disconnect batteries before probing injector-adjacent harness sections to avoid accidental shorts.
Prevention Tips
- Inspect and blow out SCR pressure sense lines at every aftertreatment PM interval.
- Apply dielectric grease to the sensor connector after cleaning corrosion.
- Route harness looms away from hot exhaust brackets to prevent insulation melt and short-to-power faults.
- Address DEF quality issues promptly to reduce soot loading in sense tubes.
Scania Field Notes for SPN 4358 FMI 3
- Torque scr catalyst exhaust differential pressure mounting fasteners per Scania 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 Scania DC13 — cold-soak faults on scr catalyst exhaust differential pressure often clear after warm-up when moisture is the root cause.
- Compare this unit’s repair history: repeat SPN 4358 FMI 3 within 5,000 miles indicates incomplete prior repair, not a new component failure.
- If scr catalyst exhaust differential pressure was recently replaced, verify correct part number for engine serial — superseded calibrations require matching hardware revision on Scania.
Related Codes
- SPN 4358 FMI 16 — often stacks with SPN 4358 FMI 3
- SPN 4360 FMI 3 — often stacks with SPN 4358 FMI 3
- SPN 4364 FMI 18 — often stacks with SPN 4358 FMI 3
FAQ
What does Scania SPN 4358 FMI 3 mean?
It means the ECM sees SCR catalyst differential pressure sensor voltage above the calibrated maximum. This is an electrical circuit fault, not a confirmed high backpressure reading.
Can I drive with SPN 4358 FMI 3 active?
Most Scania trucks continue to operate normally short term, but SCR monitor data may be unreliable and inducement timers can start if the fault is ignored for extended periods.
How do I test the Scania SCR differential pressure sensor?
Measure 5V reference and signal voltage at the connector with key on, engine off. Signal above 4.5V with vented sense lines indicates sensor or harness failure. Compare live kPa to manometer readings under load.
What does SPN 4358 FMI 3 mean on Scania DC13?
SPN 4358 FMI 3 means aftertreatment 1 scr catalyst exhaust gas differential pressure - voltage above normal. The ECM detected scr catalyst exhaust differential pressure outside calibrated limits. Verify with freeze-frame data before clearing.
Can I drive with SPN 4358 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 4358 FMI 3?
DIY repairs start at $40–$150 for wiring or filters. Shop repairs with parts run $80 - $450. Dealer pricing runs $350 - $750.
What is the most common cause of SPN 4358 FMI 3?
SCR differential pressure sensor internal short to power — confirm with visual inspection and live data before ordering parts.
Sources
- SAE J1939-71 — SPN 4358 parameter definition
- Scania OEM service information — verify by engine serial
- Mitchell1 ProDemand — 2026 labor and parts cost surveys