Truck Fault Codes

Detroit DD13 SPN 3226 FMI 16 �?NOx Sensor Upstream Above Normal

SPN 3226 FMI 16 on a Detroit DD13 means the upstream NOx sensor before the SCR catalyst is reading above the expected range. This can indicate high engine-out NOx from an EGR or combustion fault, or it may indicate a sensor drift or failure reading artificially high.

Code Details

Structured details for SPN 3226 FMI 16
Display codeSPN 3226 FMI 16
SPN3226
FMI16
OEM codeNone listed
ManufacturerDetroit Diesel
SystemAftertreatment �?SCR system
ComponentUpstream NOx sensor
Source addressUnknown or not applicable
Severitymedium
Review statussource backed
Source confidencemedium
Last reviewed2026-06-11

Plain-English Meaning

The upstream NOx sensor measures how much NOx is entering the SCR catalyst from the engine. If this reading is too high, it means either the engine is producing more NOx than expected (possibly from an EGR fault) or the sensor itself is reading incorrectly. Either condition affects the accuracy of the SCR efficiency calculation.

SPN 3226 is Aftertreatment 1 Intake NOx (upstream NOx) in J1939. FMI 16 indicates the measured value is valid and above the normal operating range. On Detroit DD13 MCM2.0, high upstream NOx can result from reduced EGR flow (EGR valve fault, cooler bypass, low EGR rate) or sensor drift. A high upstream reading combined with a normal downstream reading would yield a calculated efficiency that appears very high; a high upstream reading combined with a high downstream reading indicates actual high NOx passthrough. DiagnosticLink can display both sensors simultaneously for comparison.

Common Symptoms

  • Check engine lamp active
  • Possible SCR system warning if the NOx reading is affecting efficiency calculations
  • May accompany EGR-related fault codes if EGR is underperforming

Possible Causes

Possible causes may include the items below. The list is not a parts diagnosis.

  • EGR valve fault reducing EGR flow and raising engine-out NOx above normal levels
  • EGR cooler bypass fault allowing higher combustion temperatures and elevated NOx
  • Upstream NOx sensor drift causing a falsely elevated reading
  • Wiring or connector fault at the upstream NOx sensor
  • Combustion anomaly from fuel system or charge air issues affecting NOx production

First Checks

  • Connect DiagnosticLink and check all active codes �?EGR faults alongside SPN 3226 FMI 16 point to actual elevated NOx from EGR underperformance
  • Compare upstream and downstream NOx sensor readings simultaneously in DiagnosticLink live data
  • Check EGR valve operation using DiagnosticLink service routines
  • Inspect the upstream NOx sensor wiring and connector for damage
  • If no companion EGR faults are present and upstream reads high consistently, investigate sensor drift

Can I Keep Driving?

The vehicle is typically driveable with this fault active. If an EGR fault is the underlying cause, the engine may produce elevated NOx and potentially accelerate DPF loading. Resolve the companion EGR faults if present to reduce NOx production and maintain aftertreatment system performance.

SPN 5246 FMI 31Aftertreatment �?SCR system · SCR catalyst / NOx sensorsSPN 3364 FMI 1Aftertreatment �?DEF system · DEF quality / concentration sensor

Related Lookup Pages

Sources

  • Detroit Diesel Service and Diagnostic Resources — Public Reference Detroit Diesel (Daimler Truck North America) · oem · accessed 2026-06-11 · confidence medium

    Source: Detroit Diesel (Daimler Truck North America), Detroit Diesel Service and Diagnostic Resources — Public Reference. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • DiagnosticLink Detroit Diesel Corporation · oem · accessed 2026-05-05 · confidence medium

    Source: Detroit Diesel Corporation, DiagnosticLink. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source
  • SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium

    Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.

    Open source

FAQ

How do I tell if SPN 3226 FMI 16 on the DD13 is from an EGR fault or a faulty sensor?

The key indicator is whether companion EGR fault codes are also present. If EGR valve faults, cooler faults, or EGR position codes are active alongside SPN 3226 FMI 16, the elevated NOx reading is likely real �?caused by reduced EGR flow. If no EGR faults are present and the upstream NOx reading is consistently high under all operating conditions, sensor drift is the more likely cause.

Will SPN 3226 FMI 16 on the DD13 cause the truck to fail a roadside emissions inspection?

An active check engine lamp resulting from this fault would raise concerns in a roadside inspection. The underlying NOx issue may also affect snap-idle smoke opacity tests used in some jurisdictions. Addressing both the fault code and the underlying cause is important for emissions compliance.

Can the EGR valve on the Detroit DD13 be cleaned to resolve high NOx and SPN 3226 FMI 16?

If the EGR valve is sticking in a partially closed position due to carbon buildup, cleaning the valve bore and disc may restore normal EGR flow and reduce upstream NOx. This is more likely to be helpful if EGR position fault codes are also active. DiagnosticLink can command the EGR valve to open and close to assess whether it responds correctly after cleaning.