SPN 3226 FMI 20 — Aftertreatment Outlet NOx – Data Drifted High — Detroit DD15

SPN 3226 FMI 20 on Detroit DD15 (2010–present) means the outlet NOx sensor reading has drifted high. The ACM/MCM/CPC reports the fault via J1939. Diagnose…

Updated October 15, 2026

What it means

The outlet (tailpipe-side) NOx sensor reading has drifted high. On Detroit DD15 (2010–present) this is reported by the MCM/ACM/CPC via J1939 SPN/FMI and affects the NOx sensors system.¹

Most likely causes

1. Failing outlet NOx sensor (sensor drift / internal bias) — The outlet NOx sensor is reading high relative to expected values. Typical on this platform for SPN 3226 FMI 20.¹
2. Exhaust leak upstream of the outlet NOx sensor — A leak between the SCR and the outlet sensor can dilute or alter exhaust composition, distorting the NOx reading. Typical on this platform for NOx sensor codes.¹²
3. DEF dosing / SCR condition affecting measured NOx — DEF dosing problems or SCR performance issues can change actual outlet NOx, leading the ACM to see a high drift. DEF-related influences are typical for aftertreatment NOx readings on DD15.¹³

How to diagnose

1. Verify freeze-frame and confirm SPN 3226 FMI 20 is active on the ACM/MCM/CPC and note engine load, speed, temp at set.
2. Inspect outlet NOx sensor connector, wiring and harness for chafing, corrosion or moisture; check reference and heater circuits for plausibility.
3. Compare inlet vs outlet NOx values at steady cruise and during a controlled load; a high outlet reading with plausible inlet suggests sensor drift or exhaust influence.
4. Inspect exhaust system from SCR outlet to tailpipe for leaks, cracks or loose clamps upstream of the outlet NOx sensor.
5. Review recent DEF quality, level, dosing quantity and any SCR efficiency codes (e.g., SPN 4364) that could influence outlet NOx.
6. If no wiring/exhaust/DEF influence is found, test/replace the outlet NOx sensor and re-evaluate SPN 3226 FMI 20.

Specs and test values

Common mistake to avoid

Replacing sensors, the doser or the SCR catalyst before testing the DEF. Old, heat-stored or diluted DEF loses strength and sets the same efficiency codes. Check it with a refractometer first. Source: Ram service bulletin 25-005-14 (Diesel Exhaust Fluid).

Parts and tools you may need

• Diagnostic software for Detroit MCM/ACM/CPC (J1939 live data)
• Digital multimeter
• Inspection mirror and light
• Exhaust back-pressure/ leak test equipment
• NOx sensor socket and replacement outlet NOx sensor

Tools you’ll need

• J1939 diagnostic scan tool — Reads SPN 3226 FMI 20 from ACM/MCM/CPC and captures freeze-frame data for load, speed, and temperature at set.
• Digital multimeter — Checks outlet NOx sensor heater and reference circuits for continuity, resistance, and voltage plausibility per Detroit specs.
• Exhaust gas analyzer — Compares actual tailpipe NOx to sensor-reported outlet NOx to confirm sensor drift versus real exhaust condition.
• Inspection mirror and flashlight — Allows visual check of exhaust from SCR outlet to tailpipe for leaks, cracks, or loose clamps upstream of the outlet NOx sensor.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If inducement/derate is active, or if NOx values remain implausible after sensor and exhaust checks, have a Detroit-certified technician perform ACM-guided sensor tests and SCR efficiency evaluation. Do not attempt high-pressure fuel system service. If you suspect DEF contamination or SCR catalyst issues, professional service is required.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources