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DiscountTruck Repair

SPN 3216 FMI 16

Caution

SCR Inlet NOx Sensor — Reading Above Normal Range

SAE J1939 · heavy duty · Freightliner, Volvo Trucks, PACCAR (Kenworth / Peterbilt), International, Mack, Cummins, Western Star

Caution

Get it looked at

The NOx sensor before the SCR catalyst is reporting more NOx than the engine should be making. Usually a drifting sensor or an EGR problem upstream — not the catalyst itself.

DrivingYes. On its own this is a caution-level fault with no immediate derate. But it commonly precedes SCR efficiency and inducement faults, so treat it as an appointment to make this week, not a warning to clear and forget.

Typical repair cost

Low
$925
Typical
$1,975
High
$3,025
Parts
$800–$2,500
Labor
1–2.5 hr × $125–$210/hr = $125–$525

Provisional figures — pending verification against our receipt dataset.

Labor rate: provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these ranges.

Need it fixed

Phone-verified shops, with the date we last called each one.

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What SPN 3216 FMI 16 means

SPN 3216

SPN 3216 is aftertreatment 1 intake NOx, measured by the sensor mounted upstream of the SCR catalyst. Its job is to tell the dosing controller how much NOx is arriving so it can calculate how much DEF to inject. Every downstream efficiency calculation depends on this number being right.

FMI 16

FMI 16 means data valid but above normal operating range, moderately severe level. The signal is electrically sound and plausible — it is simply higher than the ECM expects for the current load, speed, and EGR position.

Read together, SPN 3216 FMI 16 is the ECM saying: scr inlet nox sensor — reading above normal range. The SPN identifies which parameter, the FMI identifies how it failed. Two codes sharing an SPN but carrying different FMIs are different faults with different repairs, which is why the SPN alone is never enough.

Common causes

  • Inlet NOx sensor drift — these sensors age and read progressively high
  • EGR valve stuck closed, sticking, or sooted so that actual EGR flow is below commandedLess EGR means genuinely more engine-out NOx. The sensor may be telling the truth.
  • EGR cooler restriction or leak
  • Exhaust leak upstream of the sensor drawing in ambient air
  • Turbo / VGT actuator fault changing the boost and EGR balance
  • Sensor harness chafe or connector corrosion causing signal offset

No frequency data yet. We have fewer than 5 verified repair receipts for SPN 3216 FMI 16, so we are not going to print percentages. Causes above are listed in no particular order. Had this fixed? Tell us what it turned out to be.

Technician note pending

A credentialed technician annotation for SPN 3216 FMI 16 has not been commissioned yet. Until it is, this page stays a draft and is excluded from our sitemap. We do not publish invented first-hand experience. How we verify content.

How a technician diagnoses it

  1. 1

    Compare inlet and outlet NOx sensor readings at key-on before the engine starts. Both sit in ambient air, so they should agree closely. A large split at rest condemns one of them.

    Tool: OEM service tool · Spec: TODO(verify): acceptable key-on delta per OEM

  2. 2

    Read commanded versus actual EGR position and EGR differential pressure at steady cruise. A gap here points upstream of the aftertreatment entirely.

    Tool: OEM service tool

  3. 3

    Inspect the exhaust from the turbo outlet to the sensor boss for leaks — soot tracks at joints and clamps are the tell.

  4. 4

    Check the sensor harness where it crosses the frame and near heat sources. These harnesses fail from heat and vibration more often than the sensing element does.

  5. 5

    Look for stored EGR, VGT, or boost codes and resolve those first. An inlet NOx fault set alongside an EGR fault is usually the EGR fault talking.

  6. 6

    Only after ruling out EGR and exhaust integrity, replace the inlet NOx sensor. Torque to spec and use anti-seize sparingly — these seize into the boss.

    Spec: TODO(verify): torque spec per OEM

Parts and labour

Parts and labour ranges for SPN 3216 FMI 16
PartPart numberParts cost
SCR inlet NOx sensor and harness assemblyTODO(verify)$350–$900provisional
EGR valve (if implicated)TODO(verify)$450–$1,600provisional
Labour1–2.5 hr$125–$525

Labour priced at provisional national placeholder — TODO(verify), not yet from our collected dataset. How we build these numbers.

Can I keep driving?

Yes. On its own this is a caution-level fault with no immediate derate. But it commonly precedes SCR efficiency and inducement faults, so treat it as an appointment to make this week, not a warning to clear and forget.

Find a shop that handles this

We call every shop before it appears here, and publish the date.

Sample records only — seeded to demonstrate the directory. None of these numbers has been called.

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