Mack · SCR Outlet Gas Temperature SensorHigh

Mack SPN 4363 FMI 1: SCR Outlet Temperature Below Normal

SPN 4363 FMI 1 (FMI 1) — SCR outlet gas temperature reading is below normal operating range during active regeneration — FMI 1 indicating sensor failure, exhaust leak, or impaired regen process

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Quick Answer

SPN 4363 FMI 1 on a Mack means the SCR outlet gas temperature is reading below normal range. Compare inlet vs outlet temperatures during regen with Jaltest before replacing the sensor.

Overview: Mack SPN 4363 FMI 1

When a Mack MP8 or MP7 truck logs SPN 4363 FMI 1, the aftertreatment control module has determined the SCR outlet gas temperature is below the expected range for current operating conditions. Unlike SPN 4363 FMI 3 (voltage above normal), FMI 1 means the reading is in a valid but below-normal range — which on a hot engine under load or during active regen points to sensor failure rather than actual cold exhaust. At ProfessionalDieselRepair.com, the most reliable diagnostic step is comparing the SCR inlet temperature (SPN 4360) against the outlet (SPN 4363) using Jaltest during a commanded stationary regen. If the inlet climbs normally but the outlet stays near ambient temperature, the outlet sensor has failed. If both stay low, the regen process itself is incomplete due to a DEF or fuel doser issue.

Repair Snapshot

Time

1 - 3 Hours

Difficulty

Intermediate

Parts Cost

$60 - $200

Dealer Labor

$400+

Tools Required

  • Jaltest Trucks Diagnostic Tool
  • Diesel Laptops
  • Digital Multimeter (DVOM)
  • Electrical Contact Cleaner
  • Connector Seal Kit

What SPN 4363 FMI 1 Means

SPN 4363 FMI 1 on a Mack MP8 or MP7 engine indicates the SCR outlet gas temperature sensor is reporting a temperature below the normal range expected during current engine operation. The SCR outlet sensor monitors exhaust leaving the SCR catalyst and is used to verify regeneration completion and SCR thermal performance. During active regen, the outlet should reach 400 to 600 degrees C; FMI 1 means the reading is below an expected minimum (approximately 150 degrees C) during conditions where higher temperature is expected. Jaltest and Diesel Laptops provide effective access to the Mack MP8 aftertreatment parameter set for diagnosis.

Symptoms Drivers Notice

  • SCR outlet temperature reads below 150 degrees C during a commanded regeneration event.
  • Active regeneration fails to complete, leaving the DPF at elevated soot load.
  • Amber aftertreatment lamp and possible derate warning if the incomplete regen cycle accumulates.
  • Diesel Laptops or Jaltest live data shows SPN 4363 value significantly lower than SPN 4360 (inlet) during regen.
  • Mack instrument cluster may display a service aftertreatment system message.

Most Likely Root Causes

Failed NTC thermistor in the SCR outlet sensor (open or degraded)

Common

The thermistor element degrades from long-term exposure to high-temperature exhaust gases. An open thermistor causes the ACM to read an implausibly low temperature, setting FMI 1.

DEF or exhaust condensation contaminating the sensor connector

Common

The SCR outlet is downstream of the DEF dosing point. DEF vapor or condensation migrates into the sensor connector, creating partial grounds that pull the signal below the FMI 1 threshold.

Exhaust system air leak upstream of the SCR outlet sensor

Occasional

A gasket failure or loose clamp between the SCR catalyst outlet and the sensor mounting point allows ambient air to cool the exhaust stream near the sensor, producing a genuinely low temperature reading during light-load operation.

Incomplete regeneration lowering actual SCR outlet temperature

Occasional

If a fuel doser or DEF dosing fault prevents the regen from reaching full temperature, the outlet sensor may read correctly but below the expected regen temperature, triggering FMI 1 as a secondary fault.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR outlet sensor without comparing inlet vs outlet temperature

Why it happens: A parts-cannon approach replaces the sensor without confirming whether the fault is in the sensor or in the regen process itself, which requires the inlet-versus-outlet temperature comparison.

Do this instead: Use Jaltest to monitor both SPN 4360 (inlet) and SPN 4363 (outlet) simultaneously during a commanded regen. If inlet is normal and outlet is low, the outlet sensor has failed.

Attributing FMI 1 to a catalyst efficiency problem

Why it happens: Technicians familiar with SPN 4364 (NOx efficiency) faults may assume FMI 1 on the outlet temperature sensor means the catalyst is degraded and needs replacement.

Do this instead: A degraded catalyst affects NOx conversion but does not necessarily cause the outlet temperature sensor to read below normal. Verify the sensor circuit first.

Confusing SPN 4363 FMI 1 with SPN 4363 FMI 3

Why it happens: Both involve the same component but different FMI codes require different diagnostic approaches. FMI 3 is a high-voltage fault; FMI 1 is a below-normal range fault.

Do this instead: Always confirm the FMI number before beginning diagnosis. FMI 1 — measure sensor resistance and compare inlet vs outlet temperature. FMI 3 — trace the signal circuit for a short to supply voltage.

Master Tech Commentary

Mechanic's Notes

SPN 4363 FMI 1 differs from SPN 4363 FMI 3 (high voltage, which was documented in a prior run). FMI 1 is a below-normal range fault meaning the temperature reading is plausible but lower than expected for the operating condition. The most important diagnostic step is comparing SPN 4360 (SCR inlet) against SPN 4363 (SCR outlet) during a commanded regen: if the inlet is climbing normally but the outlet stays low, the outlet sensor is failed. If both are low, the regen process itself may be incomplete due to a DEF dosing issue. Always use Jaltest for Mack MP8 aftertreatment work — it provides full bi-directional regen commands and sensor parameter access. Diesel Laptops is an effective complement for J1939 live data capture. Full sensor R&R procedure for the Mack MP8 SCR outlet is at ProfessionalDieselRepair.com.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect Jaltest Trucks diagnostic software to the Mack 9-pin diagnostic port. Navigate to the aftertreatment module data and command a stationary regeneration test. Monitor SPN 4363 (SCR outlet temperature) during the regen cycle. If the outlet temperature fails to climb above 200 degrees C while the SCR inlet (SPN 4360) is reading above 350 degrees C, the outlet sensor circuit is suspect.
  2. Step 2: After the regen test with the engine off and exhaust cooled for at least 45 minutes, disconnect the SCR outlet temperature sensor connector. Measure resistance between the sensor pins. For Mack MP8 NTC thermistors, expect 50 to 120 kOhm at ambient temperature (25 degrees C). An open reading (OL) or a reading above 500 kOhm confirms thermistor failure. Use Diesel Laptops with a J1939 adapter for a pre-test and post-test parameter comparison.
  3. Step 3: If resistance is in spec, inspect the connector for DEF crystallization, corrosion, or physical damage. The SCR outlet sensor is located downstream of the DEF injection point, where DEF vapor can migrate into the connector cavity. Clean with electrical contact cleaner and re-test resistance.
  4. Step 4: Back-probe the signal pin to sensor ground with the connector attached and key on (engine off). The voltage should reflect the current ambient temperature on the NTC curve. A signal below 0.2 V with a confirmed 5 V reference on the supply pin indicates a short-to-ground path in the connector or harness.
  5. Step 5: If the connector is clean and the harness is intact, replace the SCR outlet temperature sensor. Use Jaltest to command another stationary regen and confirm the outlet temperature rises to the expected 400 to 600 degrees C range during peak regen activity.

Live Data & Freeze-Frame Tips

With Jaltest or Diesel Laptops connected to the Mack diagnostic port, navigate to aftertreatment live data and monitor SPN 4363 (SCR outlet temperature) alongside SPN 4360 (SCR inlet temperature) and engine load. During a stationary regen command, the outlet temperature should reach 400 to 600 degrees C at peak regen. If the inlet climbs to above 350 degrees C but the outlet stays below 150 degrees C, the outlet sensor circuit is at fault. A normal inlet and a low outlet reading together is the definitive pattern for SPN 4363 FMI 1.

Safety Warnings

  • Allow the SCR canister and exhaust piping to cool for at least 45 minutes before handling the outlet temperature sensor — regen temperatures can exceed 600 degrees C.
  • Do not command a stationary regen in an enclosed space — regen produces high exhaust temperatures and concentrations of exhaust gas.
  • Wear heat-resistant gloves and eye protection when working near the aftertreatment system.
  • Ensure the DEF system has been depressurized before disconnecting any DEF line connectors in the SCR section.

When to Limp In vs Tow

SPN 4363 FMI 1 alone is rarely a tow-now situation. However, if active regen fails repeatedly and DPF soot load is elevated (confirmed by Jaltest), reduce highway driving and schedule a service appointment promptly to prevent a forced parked regen situation.

Prevention Tips for Fleets & Owner-Operators

  • Inspect the SCR outlet temperature sensor connector for DEF vapor deposits at every DPF service interval.
  • Check for exhaust leaks at the SCR outlet connection during PM inspections — an air leak can cause genuine low-temperature readings.
  • Monitor SCR outlet temperature trends in Jaltest live data to catch gradual sensor drift before an FMI 1 fault develops.
  • Replace the sensor connector seal if any DEF contamination is found in the connector cavity.

Related Fault Codes

SPN 4363 FMI 3

FMI 3 on the SCR outlet temperature sensor indicates a high-voltage circuit fault — the opposite condition from FMI 1 which is a below-normal reading.

SPN 4360 FMI 1

SPN 4360 SCR inlet temperature FMI 1 — if both inlet and outlet show FMI 1, suspect a common sensor supply circuit failure or shared harness damage.

SPN 4364 FMI 18

SPN 4364 SCR NOx conversion efficiency low may accompany SPN 4363 FMI 1 if the regen failure results in reduced SCR catalyst temperature and performance.

Frequently Asked Questions

What does Mack SPN 4363 FMI 1 mean?

SPN 4363 FMI 1 means the SCR outlet gas temperature sensor is reading below its normal operating range. On a hot engine or during active regen, this indicates sensor failure rather than actual cold exhaust.

What temperature should the SCR outlet read during regen on a Mack MP8?

During peak active regeneration on a Mack MP8, the SCR outlet temperature should reach 400 to 600 degrees C. A reading below 150 degrees C during a commanded regen event is the FMI 1 trigger.

How do I distinguish a sensor fault from an incomplete regen?

Monitor SCR inlet temperature (SPN 4360) alongside the outlet (SPN 4363) in Jaltest during a commanded regen. If the inlet climbs to above 350 degrees C but the outlet stays near ambient, the outlet sensor is failed. If both stay low, the regen process is incomplete.

Can I drive with Mack SPN 4363 FMI 1 active?

Yes, briefly. The code alone does not typically trigger an immediate derate. However, repeated failed regens will elevate DPF soot load and eventually trigger a parked regen requirement, so address the fault promptly.

Which tools diagnose SPN 4363 on a Mack MP8?

Jaltest Trucks provides full Mack MP8 aftertreatment diagnosis including bi-directional regen commands and simultaneous SCR inlet and outlet temperature monitoring. Diesel Laptops with a J1939 adapter is an effective complement for live data capture.

How much does it cost to fix Mack SPN 4363 FMI 1?

A replacement SCR outlet temperature sensor costs $60 to $200 for OEM or quality aftermarket. If the fault is connector contamination only, cleaning costs nothing. Dealer labor for diagnosis and replacement typically runs $300 to $500.