Detroit SPN4364FMI1 Fault Code: This Code Sets When the Aftertreatment Control Module (ACM) Detects Poor Selective Catalyst Reduction (SCR) NOx Conversion Efficiency
Also called SCR NOx Conversion Efficiency Very Low, This Code Sets When the Aftertreatment Control Module (ACM) Detects Poor Selective Catalyst Reduction (SCR) NOx Conversion Efficiency., This code sets when the Aftertreatment Control Module (ACM) detects poor Selective Catalyst Reduction (SCR) nox conversion efficiency, This fault code sets when the Aftertreatment Control Module (ACM) detects that the NOx conversion is lower than a calibrated threshold
This Code Sets When the Aftertreatment Control Module (ACM) Detects Poor Selective Catalyst Reduction (SCR) NOx Conversion Efficiency · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 4364 FMI 1 sets when the Aftertreatment Control Module detects that the Selective Catalyst Reduction (SCR) system is not converting NOx at an acceptable rate. The MIL and CEL come on and the engine will derate 25% until the fault clears. This is an emissions fault, not an immediate mechanical emergency, but it will limit your engine's power and needs to be run down promptly.
Medium severity. The engine keeps running but derates power by 25%, which affects productivity and drivability until diagnosed. It is not an immediate safety shutdown, but ignoring it risks repeat derates and possible sensor or doser damage.
What does Detroit error code SPN4364FMI1 mean?
SPN 4364 FMI 1 is set by the Aftertreatment Control Module (ACM) when it compares readings from the SCR inlet NOx sensor and the SCR outlet NOx sensor and finds that the SCR catalyst is not converting NOx as efficiently as it should. In plain terms, the exhaust going into the SCR catalyst still has close to the same amount of NOx coming out the other side, meaning the Diesel Exhaust Fluid (DEF) dosing and catalyst reaction are not doing their job.
This matters because SCR NOx conversion is required to meet emissions regulations. When conversion efficiency falls below the calibrated threshold, the ACM assumes the aftertreatment system cannot clean the exhaust properly and responds by derating engine power 25% as a way to force the operator to address the problem.
The root cause can be anything from bad or contaminated DEF, DEF dosing hardware problems, DEF quantity or pressure issues, drifted or failed NOx sensors, or a genuinely under-performing SCR catalyst. Several versions of this diagnostic also note that other codes in the air management, EGR, fuel, NOx sensor, or aftertreatment temperature/pressure systems should be ruled out first because those systems can all affect SCR performance indirectly.
What triggers a Detroit SPN4364FMI1 code?
The ACM monitors this fault continuously once enabling conditions are met. Enabling conditions include DEF dosing enabled, engine speed of 1000 to 2100 rpm (one version narrows this to 1000 to 1500 rpm), engine load of 15 to 100%, and non-regeneration conditions present. One version also specifies SCR inlet temperature below 650°C (1200°F), ambient temperature above 0°C (32°F), and coolant temperature above 70°C (158°F). The fault requires these conditions to be sustained for a typical duration of 30 minutes before the code sets.
Common causes of SPN4364FMI1
- Contaminated DEF (diesel fuel or oil contamination) confirmed by test strip or visual inspection
- DEF urea concentration out of range (should be 31 to 34 percent, or 28 to 36% per one version) which reduces the chemical reaction efficiency in the SCR catalyst
- DEF crystallization buildup around the DEF dosing unit gasket restricting proper dosing
- Incorrect DEF dosing quantity, outside the expected 108 to 132 mL (3.65 to 4.46 oz) range in one procedure or 102 to 138 mL (3.7 to 4.5 oz.) in another
- DEF pressure sensor reading not matching the barometric pressure sensor reading within 29.6 kPa (4.3 psi)
- Faulty or drifted SCR inlet or SCR outlet NOx sensor, confirmed when readings differ by more than 50 Parts Per Million (PPM) during a performance check
- A drifted SCR temperature sensor, when SCR inlet temperature is more than 38°C (68.4°F) lower than SCR outlet temperature during the last seven minutes of a parked regeneration log, or inlet/outlet temperatures are not within 25°C (45°F) of each other during a low temperature aftertreatment check
- Underlying issues in other emission-related systems (air management including Intake Throttle Valve, Intake Manifold Pressure, Intake Air Temperature, turbocharger; EGR; fuel system) that are not directly SCR faults but degrade NOx conversion indirectly
- A genuinely degraded SCR catalyst if all sensors, DEF quality, and dosing hardware check out but NOx conversion efficiency still fails the Parked SCR Efficiency Test
How to troubleshoot Detroit SPN4364FMI1: first checks
- Connect DiagnosticLink and check for any other active fault codes first, especially air management (Intake Throttle Valve, Intake Manifold Pressure, Intake Air Temperature, turbocharger), EGR, fuel system, other NOx sensor codes, DEF metering/pump/air valve codes, and ACM temperature or pressure codes. Repair those first since they can cause or mask this fault
- With the ignition off, use a DEF test strip (part A0005850202) to check for diesel fuel or oil contamination in the DEF, and inspect visually for cloudiness or discoloration
- Using a refractometer from the DEF test kit, verify the DEF urea concentration falls within the specified range (around 31 to 34 percent, or up to 28 to 36% depending on the emissions platform)
- Inspect around the DEF dosing unit gasket for DEF crystallization buildup, noting that some crystallization in the doser port hole is considered normal
- Run the DEF quantity test service routine in DiagnosticLink and confirm the dispensed amount falls in the expected range
- Compare the DEF pressure sensor reading against the barometric pressure sensor reading and confirm they are close (within 29.6 kPa / 4.3 psi on the platform that specifies this)
- With the DEF dosing unit electrical connector disconnected, run a parked regeneration and monitor SCR inlet and outlet NOx sensor readings for at least 30 minutes, sometimes needing 30 minutes or longer for the outlet sensor to activate, checking that inlet and outlet readings stay within 50 PPM of each other
How the code clears
There is no simple key-cycle reset for this code. The documented path is to work through the full troubleshooting sequence: rule out related fault codes, check DEF quality and quantity, check for DEF crystallization, verify NOx sensor agreement with the DEF doser disconnected during a parked regeneration, then reconnect the doser and run a parked regeneration while monitoring NOx conversion efficiency. If efficiency comes back above the specified threshold (85% on newer platforms, 70% on the EPA10 platform), the final step is to run a Parked SCR Efficiency Test through DiagnosticLink, which is what actually clears the regulatory fault. Note that disconnecting the DEF dosing unit connector during testing will itself set a DEF doser circuit fault code (SPN 3361/FMI 5 is named on one platform); that code clears on its own once the connector is reconnected and cleared in DiagnosticLink. If the NOx verification test still fails after ruling out DEF quality, quantity, and sensor agreement, the documented step is to replace the SCR inlet NOx sensor and retest before condemning the catalyst itself.
Frequently asked questions
What does SPN 4364 FMI 1 mean on a Detroit engine?
It means the Aftertreatment Control Module has detected that the SCR system is not converting NOx efficiently enough, based on comparing the SCR inlet and outlet NOx sensor readings. The engine will derate power by 25% until the issue is fixed and verified.
Will this code shut my truck or equipment down?
No, it triggers a power derate of 25% rather than a shutdown, but the reduced power will affect performance until you diagnose and clear the fault.
Can bad DEF cause this fault?
Yes. Contaminated DEF, or DEF with a urea concentration outside the specified range (about 31 to 34 percent, or 28 to 36% depending on platform), is one of the most common documented causes and should be checked early with a test strip and refractometer.
Do I need to replace the NOx sensors to fix this?
Not necessarily. The documented procedure checks DEF quality, DEF quantity, DEF pressure, and sensor agreement first. NOx sensor replacement, specifically the inlet sensor, is only called for if the NOx verification test still fails after all other checks pass.
How is this code actually cleared?
After repairs, DiagnosticLink is used to run a parked regeneration and confirm NOx conversion efficiency is above the required threshold, then a Parked SCR Efficiency Test is performed. That test is what clears the regulatory fault, not a simple code erase.
Why did disconnecting the DEF doser connector set a new fault code?
Running the engine with the DEF dosing unit electrically disconnected is part of the test procedure to isolate NOx sensor readings, and it deliberately induces a DEF doser circuit fault (SPN 3361/FMI 5 on one platform). That code clears once you reconnect the connector and clear it in DiagnosticLink.
What other fault codes should I check before working on this one?
Check for and repair any codes related to the Intake Throttle Valve, Intake Manifold Pressure sensor, Intake Air Temperature sensor, turbocharger, EGR system, fuel system, other NOx sensors, and aftertreatment temperature or pressure sensors first, since these can cause or contribute to poor SCR conversion efficiency.