Scania · SCR Catalyst Exhaust Differential PressureHigh

Scania SPN 4358 FMI 3: SCR Catalyst Exhaust Differential Pressure Voltage High

SPN 4358 FMI 3 (FMI 3) — Aftertreatment 1 SCR Catalyst Exhaust Gas Differential Pressure - Voltage Above Normal

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Quick Answer

SPN 4358 FMI 3 on Scania means the SCR catalyst differential pressure sensor circuit voltage is above the ECM calibrated maximum. Check the 5V reference, signal wire, and sense tubes before replacing the SCR catalyst.

Overview: Scania SPN 4358 FMI 3

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.

Repair Snapshot

Time

1 - 3 Hours

Difficulty

Intermediate

Parts Cost

$80 - $450

Dealer Labor

$350 - $750

Tools Required

  • Jaltest Diagnostic System
  • Texa
  • Multimeter
  • Compressed Air

What SPN 4358 FMI 3 Means

SPN 4358 FMI 3 on Scania DC13 and DC16 engines means the ECM is receiving a differential pressure signal voltage above the calibrated maximum for the SCR catalyst exhaust pressure sensor circuit. FMI 3 is an electrical fault — not a confirmed high backpressure event — and usually traces to a shorted signal wire, failed sensor, or reference voltage bleed. The amber MIL illuminates immediately and the aftertreatment monitor may flag SCR efficiency data as unreliable until the circuit is repaired.

Symptoms Drivers Notice

  • Amber check engine or aftertreatment warning lamp illuminated
  • SCR differential pressure reading pegged high or erratic on scan tool live data
  • Possible companion SPN 4358 FMI 4 (voltage below normal) if harness is intermittently damaged
  • No immediate power derate on most Scania calibrations, but SCR inducement timers may start if fault persists
  • Freeze-frame may show implausible differential pressure values at idle (e.g., 50+ kPa when actual backpressure is near zero)

Most Likely Root Causes

SCR differential pressure sensor internal short to power

Very Common

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

Common

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

Moderate

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.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR catalyst for a voltage-high electrical fault

Why it happens: SPN 4358 references the SCR catalyst, so a parts-cannon approach assumes the brick is restricted without reading the FMI.

Do this 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

Why it happens: Techs focus only on electrical tests and miss soot-blocked sample tubes that bias sensor output.

Do this 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.

Master Tech Commentary

Mechanic's Notes

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.

Step-by-Step Diagnostic Procedure

  1. Step 1: 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.
  2. Step 2: 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.
  3. Step 3: 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.
  4. Step 4: 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.
  5. Step 5: 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.

Live Data & Freeze-Frame Tips

In Jaltest or Texa, graph SCR differential pressure voltage alongside calculated kPa at idle and 1500 RPM. At key-on engine-off with lines vented, voltage should sit near 0.5V. Values above 4.5V with vented lines confirm sensor or harness fault, not exhaust restriction.

Safety Warnings

  • 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.

When to Limp In vs Tow

SPN 4358 FMI 3 alone does not require towing on most Scania trucks. Tow if companion SCR inducement codes activate a severe derate or if multiple aftertreatment electrical faults appear simultaneously.

Prevention Tips for Fleets & Owner-Operators

  • 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.

Related Fault Codes

SPN 4358 FMI 16 — Aftertreatment #1 SCR Catalyst Exhaust Gas Differential Pressure Sensor

Data valid but above normal differential pressure — genuine restriction concern vs electrical FMI 3.

SPN 4360 FMI 3 — Aftertreatment #1 SCR Catalyst Intake Gas Temperature Sensor

SCR intake gas temperature voltage high — adjacent aftertreatment sensor circuit fault.

SPN 4364 FMI 18 — SCR NOx Conversion Efficiency

SCR NOx conversion efficiency low — may follow unreliable pressure data from a failed DP sensor.

Frequently Asked Questions

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.

Is FMI 3 the same as high SCR backpressure?

No. FMI 3 is voltage above normal in the sensor circuit. FMI 16 means the pressure reading is valid but above the operational threshold — a mechanical restriction concern.

Which scan tools read Scania SPN 4358 live data?

Jaltest and Texa provide full SCR differential pressure voltage and kPa on Scania DC13 and DC16 platforms. Noregon JPRO displays the fault but may have limited bi-directional control.

How do I clear SPN 4358 FMI 3?

Repair the sensor circuit or sense lines, then clear with Jaltest or Texa. The code returns on the next key cycle if voltage remains above the ECM threshold.

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