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Mercedes-Benz SPN 4360 FMI 16: SCR Intake Gas Temperature Above Normal

SPN 4360 FMI 16 (FMI 16) — Aftertreatment 1 SCR Catalyst Intake Gas Temperature - Data Valid But Above Normal Operational Range - Moderately Severe Level

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Quick Answer

SPN 4360 FMI 16 means the SCR catalyst intake gas temperature is valid but above the moderately severe limit. Investigate DOC regen behavior, DEF dosing, and exhaust oil contamination before replacing the temperature sensor.

Overview: Mercedes-Benz SPN 4360 FMI 16

The SCR intake gas temperature tells the Mercedes aftertreatment control module how much heat the catalyst brick is receiving before DEF is injected. When SPN 4360 FMI 16 sets, the ECM has confirmed exhaust temperatures at the SCR inlet exceed safe operating bounds. ProfessionalDieselRepair.com prioritizes upstream heat sources — DOC overtemperature, regen aborts, and oil in the exhaust — over immediate sensor replacement because the sensor is usually reporting accurately.

Repair Snapshot

Time

2 - 5 Hours

Difficulty

Advanced

Parts Cost

$120 - $3,500

Dealer Labor

$600 - $1,200

Tools Required

  • Jaltest Diagnostic System
  • Texa
  • Infrared Pyrometer
  • DEF Refractometer

What SPN 4360 FMI 16 Means

SPN 4360 FMI 16 on Mercedes-Benz OM471 and OM936 engines means the SCR catalyst intake gas temperature sensor is reporting a valid reading above the moderately severe high threshold. This indicates the exhaust entering the SCR brick is hotter than the ECM expects for safe urea dosing and catalyst protection. Causes range from active DPF regen heat propagation and DEF dosing anomalies to a genuinely overheating DOC upstream. Prolonged operation can trigger aftertreatment derate and SCR efficiency monitor faults.

Symptoms Drivers Notice

  • Amber or red aftertreatment warning lamp illuminated
  • Elevated SCR intake temperature on live data during normal cruise (often above 450°C when loaded)
  • Possible DEF dosing reduction or temporary urea injection cutback by the ACM
  • Companion codes for DOC outlet temperature or DPF regen status may be active
  • Reduced fuel efficiency if ECM retards timing to lower exhaust heat input

Most Likely Root Causes

DOC or DPF regen overtemperature propagating to SCR inlet

Very Common

An over-fueling HC doser or loaded DPF during active regen can push DOC outlet above 650°C, and that heat wave reaches the SCR intake within one exhaust pulse cycle.

Engine oil or fuel entering exhaust and oxidizing in the DOC

Common

Turbo seal leaks and worn cylinder liners send hydrocarbons into the DOC, creating exothermic reactions that elevate all downstream gas temperatures including SCR intake.

Faulty DEF doser causing localized SCR hot spots

Moderate

A dribbling doser that injects urea into a still-cold or overheated brick can cause ammonia slip reactions and temperature spikes at the SCR inlet sensor location.

Common Misdiagnoses (Don't Waste Parts)

Replacing the SCR intake temperature sensor without verifying actual exhaust heat

Why it happens: The fault names the sensor parameter, encouraging a parts-cannon sensor swap when live data shows all EGTs elevated together.

Do this instead: Log DOC outlet, DPF inlet, and SCR intake simultaneously. If all channels are high, chase the heat source upstream — doser, regen strategy, or oil ingestion.

Clearing the code after a regen abort without fixing the regen root cause

Why it happens: Temperatures drop after regen stops and the code appears resolved.

Do this instead: Review regen history for repeat aborts. Address HC doser flow, DPF soot load, and DEF quality before returning the truck to service.

Master Tech Commentary

Mechanic's Notes

ProfessionalDieselRepair.com checks DOC outlet and HC dosing before replacing the SCR intake temperature sensor on SPN 4360 FMI 16. A lazy DEF doser that dribbles during non-regen conditions can elevate SCR brick temperature without any engine mechanical fault. Sensor replacement alone is a parts-cannon fix when the exhaust stream is genuinely hot.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect Jaltest or Texa and log SPN 4360 alongside DOC outlet (SPN 4766), DPF inlet (SPN 3242), and turbo outlet EGT during a 10-minute road test at steady 55-65 mph.
  2. Step 2: Mercedes OM471 normal SCR intake temperature at cruise is typically 250°C-420°C depending on load; sustained readings above 480°C at moderate load confirm a genuine overtemperature event, not sensor drift.
  3. Step 3: Review regen history in the scan tool — aborted or incomplete regens that spike DOC outlet above 600°C will propagate high heat into the SCR intake within 30-60 seconds.
  4. Step 4: Inspect the DEF doser spray pattern at the SCR inlet mixer. A stuck-open doser or crystallized nozzle can cause localized hot spots; DEF crystallization appears as white chalky deposits on the mixer screen.
  5. Step 5: If all gas temperatures are genuinely high, check for engine oil consumption past the turbo seal — oil oxidation in the DOC adds uncontrolled heat. Compare SCR intake sensor to an infrared pyrometer at the sensor boss; divergence beyond 30°C suggests sensor replacement.

Professional service / OEM-level scan tools are recommended before clearing this fault for good.

Live Data & Freeze-Frame Tips

In Jaltest, plot SCR intake temperature against DEF dosing rate and NOx upstream/downstream. Normal operation shows SCR intake stabilizing below 450°C at highway cruise. A rising SCR intake with flat engine load and active DEF dosing suggests doser or catalyst efficiency problems, not a bad sensor.

Safety Warnings

  • SCR and DOC housings exceed 500°C during active regen — never touch exhaust components without confirming temperature with a pyrometer.
  • DEF vaporizes into ammonia above 200°C — avoid inhaling exhaust near an open doser test port.
  • Use OEM-spec DEF at 32.5% urea concentration verified with a refractometer.

When to Limp In vs Tow

Tow if red stop-engine lamp accompanies SPN 4360 FMI 16, if repeated regen aborts occur with visible exhaust discoloration, or if power derate limits the truck below 45 mph.

Prevention Tips for Fleets & Owner-Operators

  • Monitor regen completion rates in fleet telematics and address abort trends before SCR damage occurs.
  • Replace DEF filters and inspect doser tips at OEM-recommended aftertreatment service intervals.
  • Address turbo oil consumption early — oil-fed DOC overheating is the leading cause of cascading SCR temperature faults.
  • Use only API-licensed DEF to prevent crystallization that blocks doser spray patterns.

Related Fault Codes

SPN 4766 FMI 16 — DPF Regeneration Feedback

DOC outlet temperature high — upstream heat source for elevated SCR intake readings.

SPN 4363 FMI 16 — SCR Outlet Gas Temperature

SCR outlet temperature high — downstream confirmation of SCR brick overheating.

SPN 4364 FMI 18 — Aftertreatment SCR Catalyst

SCR NOx conversion efficiency low — often follows sustained SCR overtemperature events.

Frequently Asked Questions

What is a normal SCR intake temperature on Mercedes OM471?

At highway cruise under moderate load, SCR intake typically reads 250°C-420°C. Sustained readings above 480°C at cruise trigger FMI 16 on most calibrations.

Can a bad DEF doser cause SPN 4360 FMI 16?

Yes. A stuck-open or crystallized doser can cause incorrect urea reactions and localized hot spots that elevate SCR intake temperature readings.

Does SPN 4360 FMI 16 cause derate?

It can. Mercedes aftertreatment strategies may reduce engine torque and DEF dosing when SCR intake temperature exceeds safe limits to protect the catalyst brick.

How is FMI 16 different from FMI 0 on SPN 4360?

FMI 0 is severely high — often immediate derate. FMI 16 is moderately severe, giving more time to diagnose before the most aggressive protection actions activate.

Which tools diagnose Mercedes SCR temperature faults?

Jaltest and Texa provide full SCR temperature logging, regen history, and DEF dosing data on OM471 and OM936 engines.

Can I clear SPN 4360 FMI 16 without fixing the cause?

The code clears temporarily but returns on the next drive cycle if SCR intake temperature again exceeds the threshold. Fix the heat source first.

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