Diesel Trouble Codes

SPN 4364 FMI 18 — Aftertreatment SCR Conversion Efficiency – Below Normal, Moderately Severe — Detroit DD15

Diagnostic reference for Detroit DD15 (2010–present) with SAE J1939 SPN/FMI.

Updated October 15, 2026

What it means

SCR NOx conversion efficiency is below the expected level. On Detroit DD15 (2010–present) this is reported by the MCM/ACM/CPC via J1939 and is the most common DD15 aftertreatment code. It can trigger operator inducement (SPN 5246) if not addressed.12

Most likely causes

  1. Poor or contaminated DEF (wrong concentration, water, or degraded fluid) leading to reduced NOx conversion. — DEF quality issues are a frequent trigger for low SCR efficiency on DD15. Verify DEF meets ISO 22241 and check for contamination or age-related degradation.1
  2. DEF crystal build-up in the doser/mixer restricting proper dosing. — Crystallization from heat soak or poor atomization can reduce effective DEF delivery to the SCR catalyst.1
  3. Biased or failing NOx sensors (inlet or outlet) causing incorrect efficiency calculation. — Inlet NOx reading low (SPN 3216 FMI 18) or outlet NOx drifted high (SPN 3226 FMI 20) can skew conversion efficiency. Sensor bias, wiring, or exhaust leaks near sensors distort readings.134
  4. Exhaust leaks upstream or downstream of the SCR affecting NOx measurements. — Leaks alter sensor readings and reduce effective reductant mixing, leading to calculated low conversion efficiency.1
  5. Degraded SCR catalyst reducing conversion capability. — Aged or poisoned catalyst will not achieve expected NOx reduction even with correct DEF dosing.1

How to diagnose

  1. Confirm the code is active on Detroit DD15 via MCM/ACM/CPC J1939 data and note any accompanying SCR/NOx/DEF codes.
  2. Review recent DEF history: concentration, source, age, and any contamination events; check for operator inducement SPN 5246 indicating severity escalation.
  3. Inspect DEF dosing system for signs of crystallization or blockage at the doser/mixer and verify DEF pump/dosing operation via ACM data.
  4. Compare inlet and outlet NOx sensor readings at steady-state cruise; look for low intake NOx (SPN 3216 FMI 18) or high outlet drift (SPN 3226 FMI 20) indicating sensor bias.
  5. Check for exhaust leaks in the aftertreatment piping and around NOx sensor bungs that could dilute or alter sensor signals.
  6. Assess SCR catalyst condition via ACM efficiency data and temperature profiles; consider catalyst degradation if dosing and sensors are normal.

Specs and test values

CheckNormal readingWhen to measureSource
DEF urea concentration32.5% (check with a DEF refractometer)Sample from the tankRam service bulletin 25-005-14 (Diesel Exhaust Fluid)

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

Tools you’ll need

Rough cost and difficulty

Likely repairPartsShop laborWho usually does it
DEF system drain, flush and refill (contaminated fluid)$50–$2001–3 hrsDIY with tools and a manual
DEF injector (doser)$300–$9000.5–2 hrsDIY with tools and a manual
NOx sensor$300–$9000.5–2 hrsDIY with tools and a manual
SCR catalyst$2,500–$6,0001–4 hrsShop

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 operator inducement (SPN 5246) is active or efficiency remains low after DEF and sensor checks, have a certified Detroit technician evaluate the SCR catalyst and dosing system with OEM diagnostics to avoid unnecessary component replacement.

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

  1. Curated DTC seed — Detroit DD15 (curated) ↩︎ ↩︎ ↩︎ ↩︎ ↩︎ ↩︎

  2. Curated DTC seed — Detroit DD15 (curated) ↩︎

  3. Curated DTC seed — Detroit DD15 (curated) ↩︎

  4. Curated DTC seed — Detroit DD15 (curated) ↩︎