SPN 4364 FMI 18: SCR Efficiency Low — Causes, Testing & What to Check Before Replacing the Catalyst

SPN 4364 FMI 18: SCR Efficiency Low — Causes, Testing & What to Check Before Replacing the Catalyst

SPN 4364 FMI 18: SCR Efficiency Low — Causes, Testing & What to Check Before Replacing the Catalyst

Few aftertreatment fault codes can lead to an expensive misdiagnosis as quickly as SPN 4364 FMI 18.

A truck comes into the shop with a check-engine light and an SCR efficiency fault. The inlet and outlet NOx readings don't look right. It's tempting to assume the SCR catalyst has failed.

But that can be a very expensive assumption.

On Detroit GHG14 systems, SPN 4364 FMI 18 is described as “SCR NOx Conversion Efficiency Low.” The Aftertreatment Control Module monitors the inlet and outlet NOx sensors and sets the fault when calculated Selective Catalytic Reduction efficiency falls below the expected threshold. Detroit's own troubleshooting procedure lists contaminated DEF, incorrect DEF concentration, biased NOx sensors and excessive DEF buildup as possible causes — before an SCR assembly is condemned. NHTSA

That means:

SPN 4364 FMI 18 is an SCR system-performance fault. It is not automatically an SCR catalyst failure.

Understanding that distinction can save a shop or fleet thousands of dollars.

What Does SPN 4364 FMI 18 Mean?

The SCR system's job is to reduce nitrogen oxides — commonly referred to as NOx — in diesel exhaust.

Diesel Exhaust Fluid is injected into the exhaust stream upstream of the SCR catalyst. Through the SCR process, the urea in DEF ultimately supplies ammonia, which reacts with NOx over the catalyst and converts it primarily into nitrogen and water. Cummins describes DEF as a solution containing approximately 32.5% automotive-grade urea and 67.5% water. Cummins Inc.

To determine whether that process is actually working, many heavy-duty systems use two NOx sensors:

Inlet NOx sensor → SCR catalyst → Outlet NOx sensor

The inlet sensor tells the control system how much NOx is entering the SCR.

The outlet sensor tells it how much remains after the exhaust has passed through the SCR.

If the system sees plenty of NOx entering the catalyst but not enough reduction afterward, calculated SCR conversion efficiency falls.

Eventually, the ECM or aftertreatment controller may set SPN 4364 FMI 18.

Detroit documentation specifically identifies the SCR inlet and outlet NOx sensors as monitored parameters for this fault. NHTSA

But here's the important part:

Low calculated SCR efficiency does not necessarily mean the catalyst itself is bad.

The computer only knows what the sensors and system inputs are telling it.

If one of those inputs is wrong — or the DEF isn't being delivered correctly — the computer can see poor conversion even though the catalyst isn't the root problem.

Common Causes of SPN 4364 FMI 18

The exact diagnostic procedure varies by engine manufacturer, model year and emissions calibration. Always use current OEM service information for the specific engine you're working on.

However, there are several areas that should be investigated before replacing an SCR catalyst.

1. Poor or Contaminated DEF

Start with the fluid.

DEF needs to have the proper concentration and purity for the SCR system to operate correctly.

Cummins specifies DEF at approximately 32.5% urea and 67.5% water and notes that SCR engines are designed around this concentration. Cummins Inc.

Problems can occur if DEF has been:

  • Diluted with water
  • Contaminated with diesel fuel
  • Contaminated with oil
  • Contaminated with coolant
  • Contaminated with dirt or other chemicals
  • Improperly stored
  • Replaced with fluid that does not meet the proper specification

Detroit's published SPN 4364 FMI 18 procedure specifically tells technicians to check for diesel or oil contamination and then verify DEF concentration using a refractometer. In that particular GHG14 procedure, the acceptable diagnostic range is 31% to 34% urea. NHTSA

Do not assume the DEF is good just because the tank is full.

Test it.

2. A Biased Inlet or Outlet NOx Sensor

This is one of the most important checks.

A NOx sensor doesn't have to completely fail and set a dedicated NOx sensor circuit code to cause trouble.

It can become biased.

Imagine the actual exhaust looks like this:

Inlet: 400 ppm NOx
Outlet: 40 ppm NOx

That's a substantial reduction.

But suppose the outlet sensor has drifted and reports:

Outlet: 180 ppm

The controller now believes considerably more NOx is getting through the SCR than actually is.

Calculated SCR efficiency falls — and an efficiency fault can result.

The opposite problem can happen with an inaccurate inlet sensor.

Detroit specifically lists a biased NOx sensor among the possible causes of SPN 4364 FMI 18. Its published diagnostic procedure includes a test comparing inlet and outlet NOx sensors under controlled conditions before directing the technician toward sensor replacement. NHTSA

This is why replacing an SCR catalyst based solely on the fault code is risky.

Make sure the measuring equipment is telling the truth before replacing the component being measured.

3. DEF Isn't Being Injected Correctly

Good DEF doesn't help if the system can't deliver the correct amount of it.

A dosing problem can involve:

  • DEF pump
  • DEF dosing valve/injector
  • Plugged DEF filter
  • Restricted DEF line
  • Leaking DEF line
  • Pressure problem
  • Electrical problem
  • Damaged connector or harness
  • Blocked doser mounting port
  • Poor spray pattern

The controller expects a certain amount of DEF to enter the exhaust under specific operating conditions.

If it commands dosing but too little DEF actually reaches the SCR, ammonia production will be insufficient.

NOx conversion drops.

SPN 4364 FMI 18 can follow.

Detroit's GHG14 diagnostic procedure calls for a DEF quantity test before moving deeper into SCR diagnosis. The application-specific procedure shown in the bulletin expects 108–132 mL during its prescribed test and directs technicians toward the dosing unit if the measured quantity is outside specification. Those numbers are specific to that Detroit application and should not be treated as a universal specification for every engine. NHTSA

International's A26 supplemental procedure follows the same general logic: inspect for DEF-system leaks, inspect the dosing port, then perform a dosing-system test to verify both output quantity and spray pattern. NHTSA

The lesson is simple:

Before blaming the SCR catalyst, prove that DEF is actually reaching it correctly.

4. A Plugged or Crystallized DEF Doser

White crystallized DEF around an injector doesn't automatically mean the injector itself has failed.

Small deposits can occur in normal service.

Excessive buildup, however, can interfere with DEF delivery or distribution.

The doser mounting port can become restricted, preventing the correct spray from entering the exhaust.

International's published A26 diagnostic procedure specifically instructs technicians to remove the DEF doser and inspect its mounting surface and injection port for corrosion or clogging before proceeding to further testing. NHTSA

A restricted injector port can turn a perfectly functional pump and injector into an SCR efficiency problem simply because the fluid isn't being introduced into the exhaust properly.

5. Excessive DEF Buildup in the Mixing Chamber

There's another side to dosing problems:

Too much DEF — or DEF that isn't properly decomposing — can also cause misleading readings.

Detroit lists excessive DEF buildup in the exhaust as a potential cause of SPN 4364 FMI 18 and notes that operating patterns such as high idle time, low average speed and low drive load can contribute to accumulation. NHTSA

International has also published information describing a phenomenon known as ammonia slip.

Under certain conditions, accumulated DEF can result in ammonia passing through the SCR without reacting as intended. International notes that this ammonia can then influence the outlet NOx sensor and create a biased reading, making calculated SCR conversion appear worse than it really is. NHTSA

So a strange outlet NOx reading does not automatically mean:

“The outlet NOx sensor is bad.”

And it doesn't automatically mean:

“The SCR is bad.”

You have to determine why the outlet reading looks wrong.

6. Exhaust Leaks

Don't overlook the mechanical basics.

Leaks in the intake, exhaust or aftertreatment system can change airflow and sensor readings enough to affect emissions diagnostics.

International's A26 SPN 4364 FMI 18 procedure specifically instructs technicians to check for airflow restrictions and smoke-test the intake, exhaust and aftertreatment systems for leaks before progressing further into the diagnostic routine. NHTSA

Look carefully around:

  • Exhaust manifold connections
  • Turbocharger connections
  • Flex sections
  • V-band clamps
  • DOC/DPF connections
  • SCR inlet connections
  • Sensor bungs
  • Cracked exhaust pipes
  • Aftertreatment housings

A small leak can cause a surprisingly large diagnostic headache.

7. Other Engine or Aftertreatment Faults

SPN 4364 FMI 18 shouldn't always be diagnosed in isolation.

Detroit's published troubleshooting sequence tells technicians to address several related faults before continuing with SCR efficiency diagnosis, including faults involving:

  • Air management
  • EGR
  • Fuel system
  • NOx sensors

The reasoning makes sense.

SCR performance depends partly on what the engine sends into the aftertreatment system in the first place.

If combustion, airflow or EGR control is incorrect, engine-out NOx and exhaust conditions may differ substantially from what the aftertreatment controller expects. NHTSA

So before replacing anything:

Look at the entire fault-code list.

Don't diagnose one code while ignoring the three codes that caused it.

8. Wiring and Connector Problems

NOx sensors are electronic modules operating in one of the harshest environments on a truck.

Their harnesses are exposed to:

  • Heat
  • Water
  • Road salt
  • Oil
  • Vibration
  • Corrosion
  • Abrasion

The same applies to DEF pumps, dosing valves and other aftertreatment components.

Before replacing a sensor, inspect:

  • Connector pins
  • Pin tension
  • Corrosion
  • Water intrusion
  • Wiring near hot exhaust components
  • Harness rub-through
  • Power and ground integrity
  • CAN communication where applicable

A new sensor installed on a damaged harness doesn't fix anything.

9. Incorrect Exhaust Temperature

SCR chemistry is heavily dependent on temperature.

DEF needs sufficient exhaust heat to decompose and generate the ammonia required for NOx reduction.

If temperature sensor data is incorrect — or the exhaust simply isn't reaching the required operating window — SCR performance can suffer.

This is one reason OEM diagnostic procedures frequently evaluate SCR efficiency during controlled operating conditions or a parked regeneration rather than simply looking at two random NOx readings while the truck idles in the parking lot.

On the Detroit GHG14 example, the efficiency monitor runs under defined operating conditions with DEF dosing enabled, and the published verification procedure uses a parked regeneration. NHTSA

Context matters when interpreting NOx readings.

Comparing sensors at cold idle isn't the same thing as evaluating them when the SCR system is hot and actively dosing.

10. The SCR Catalyst Actually Is Failing

Yes — sometimes the SCR catalyst really is the problem.

Catalysts do not last forever.

SCR performance can deteriorate because of:

  • Thermal damage
  • Chemical contamination
  • Physical substrate damage
  • Oil or coolant contamination
  • Long-term aging
  • Abnormal engine operation
  • Severe deposit accumulation

But the catalyst is generally one of the most expensive components in the system.

That's exactly why it should be proven bad, rather than guessed bad.

If DEF quality is correct, dosing is correct, the injector is functioning properly, there are no relevant exhaust leaks, sensor readings are trustworthy and the catalyst still cannot achieve the OEM-required conversion efficiency under the proper test conditions, then the SCR assembly becomes a much stronger suspect.

A Practical Diagnostic Order for SPN 4364 FMI 18

Every engine manufacturer's procedure is different, but as a general diagnostic strategy, it makes sense to work from basic system checks toward the expensive components.

1. Scan the entire vehicle

Record active, inactive and pending faults.

Don't immediately clear them.

Look for faults involving:

  • NOx sensors
  • DEF pressure
  • DEF quality
  • DEF dosing
  • Exhaust temperature
  • EGR
  • Turbocharger/air management
  • Fuel system
  • DPF

Detroit and International diagnostic literature both instruct technicians to address relevant accompanying faults before proceeding deeply into SPN 4364 diagnosis. NHTSA

2. Inspect the exhaust and aftertreatment system

Check clamps, pipes, bellows, sensor mounting locations and aftertreatment housings.

Look for black soot tracks.

A soot trail around a joint is a pretty good indication that exhaust has been escaping.

3. Test the DEF

Don't eyeball it.

Use the appropriate refractometer or OEM-approved DEF test equipment.

Proper automotive DEF is approximately 32.5% urea and should meet the appropriate ISO specification. Cummins Inc.

4. Verify DEF pressure and delivery

Run the OEM's DEF pressure, dosing or quantity test.

If the commanded quantity isn't being delivered, investigate the pump, filter, lines and injector before doing anything with the catalyst.

5. Inspect the DEF doser

Look for:

  • Crystallization
  • Restricted port
  • Damaged tip
  • Corrosion
  • Poor spray pattern

6. Evaluate NOx sensor data

Compare inlet and outlet behavior under the operating conditions specified by the manufacturer.

Don't replace a NOx sensor simply because its number “looks weird.”

Determine whether the reading makes sense relative to engine operation and the other sensor.

7. Perform the OEM SCR efficiency test

This may involve:

  • Parked regeneration
  • SCR performance test
  • NOx conversion test
  • Data logging
  • Specific engine temperature/load conditions

Follow the service procedure for that engine.

8. Condemn the catalyst only after the supporting systems pass

If everything feeding and measuring the SCR is functioning correctly but conversion efficiency remains below specification, then the SCR catalyst itself becomes a legitimate suspect.

Inlet vs. Outlet NOx: What Should You See?

There isn't one universal ppm number that applies to every engine and every operating condition.

Engine load, EGR operation, exhaust temperature, DEF dosing rate and calibration all affect NOx readings.

What matters is the relationship between the sensors while the SCR is actively operating.

The basic idea is:

High NOx entering SCR → substantially lower NOx leaving SCR

If inlet and outlet NOx remain relatively close together while DEF is being properly dosed and the catalyst is at the correct operating temperature, the system isn't achieving the expected conversion.

But you still need to determine whether the problem is:

Measurement → DEF delivery → exhaust conditions → catalyst

in that order.

Detroit's diagnostic procedure illustrates this distinction particularly well: it includes a controlled comparison of inlet and outlet NOx sensor readings before the final SCR efficiency evaluation. NHTSA

Don't Shotgun NOx Sensors

This deserves its own section.

NOx sensors fail.

We sell a lot of them because they absolutely do fail.

But SPN 4364 FMI 18 does not tell you to replace both NOx sensors.

If a truck has:

  • SPN 4364 FMI 18
  • No dedicated NOx sensor faults
  • Correct DEF
  • Proper dosing
  • No exhaust leaks

then evaluating sensor bias makes sense.

But blindly installing an inlet and outlet NOx sensor, clearing the fault and hoping for the best isn't diagnosis.

The same is true of DEF pumps and dosing valves.

Parts should be replaced because testing points to the component — not because they're located somewhere inside the system mentioned by the fault code.

Don't Replace the SCR Just Because a Regen Didn't Fix It

A parked regeneration can be an important part of the diagnostic process.

Detroit's published procedure even notes that excessive DEF accumulation can sometimes be cleared through regeneration procedures and directs technicians to evaluate NOx conversion afterward. NHTSA

But a regen isn't magic.

It will not repair:

  • A biased NOx sensor
  • A failed DEF pump
  • A plugged injector
  • Incorrect DEF concentration
  • A broken harness
  • An exhaust leak
  • A physically damaged SCR catalyst

If SPN 4364 FMI 18 returns, continue diagnosing the reason conversion efficiency is low.

Can SPN 4364 FMI 18 Cause a Derate?

The answer depends on the engine, emissions generation, calibration and what other faults are present.

For example, Detroit's published GHG14 procedure lists no immediate engine reaction for the specific version documented in that bulletin, while other emissions generations and manufacturers may use different inducement strategies. NHTSA

International's A26 supplemental bulletin similarly stated that the specific condition covered by that bulletin produced a warning light without drivability problems or a derate. NHTSA

That is why statements like “SPN 4364 FMI 18 always causes a 5 MPH derate” are misleading.

Never assume the behavior of one engine or emissions calibration applies to another.

However, unresolved SCR and DEF faults can be accompanied by other emissions faults that ultimately trigger inducement strategies depending on the application.

Address the problem before it turns into a roadside issue.

One Code — Many Possible Causes

This is the biggest takeaway.

Consider the number of components involved in determining SCR efficiency:

Engine combustion → EGR/air management → exhaust temperatures → DEF tank → DEF pump → DEF lines → DEF injector → decomposition/mixing → SCR catalyst → inlet NOx sensor → outlet NOx sensor → wiring → control module

A fault at several points in that chain can make the ECM conclude:

SCR conversion efficiency is too low.

That's why SPN 4364 FMI 18 should be treated as a diagnostic starting point, not a replacement-part instruction.

The Bottom Line

If you're dealing with SPN 4364 FMI 18, don't immediately replace the SCR catalyst.

Start by answering a few questions:

  • Is the DEF clean and at the correct concentration?
  • Is the DEF pump building the correct pressure?
  • Is the system delivering the correct quantity?
  • Is the DEF doser restricted or crystallized?
  • Are there exhaust leaks?
  • Are other engine or aftertreatment faults present?
  • Are the inlet and outlet NOx sensors reporting believable values?
  • Is the SCR system reaching the conditions necessary for proper conversion?
  • Does the catalyst still fail the manufacturer's efficiency test after everything else has passed?

Only after those questions are answered should an expensive SCR assembly move to the top of the list.

Proper diagnosis takes longer than throwing parts at a truck.

But it's usually a lot cheaper.

Need a NOx Sensor, DEF Pump or DEF Dosing Valve?

At AftertreatmentParts.com, we specialize in emissions and aftertreatment components for heavy-duty trucks and diesel equipment throughout Canada and the United States.

We supply replacement:

  • Inlet and outlet NOx sensors
  • DEF pumps
  • DEF dosing valves and injectors
  • Particulate matter sensors
  • Exhaust temperature sensors
  • Differential and exhaust pressure sensors
  • DEF headers and level sensors
  • Aftertreatment wiring and related components

Our goal is simple: help repair shops, fleets and equipment owners reduce parts costs and get equipment back to work faster.

If you know the part number you need, search it directly at AftertreatmentParts.com.

If you're unsure which NOx sensor, DEF pump or dosing valve fits your application, contact us with your VIN, engine information and existing part number, and we'll help identify the correct replacement.

Sources

Detroit Diesel Corporation / NHTSA — SPN 4364/FMI 18, GHG14 diagnostic procedure. The document defines the fault as low SCR NOx conversion efficiency, identifies the inlet and outlet NOx sensors as monitored parameters, and lists contaminated DEF, improper urea concentration, sensor bias and DEF buildup among possible causes. NHTSA

Navistar / International — A26 SPN 4364 FMI 18 Supplemental Diagnostics. The procedure covers airflow and exhaust leak checks, DEF-system leak testing, doser inspection, dosing output and spray-pattern testing. NHTSA

Cummins — Diesel Exhaust Fluid information. Cummins describes DEF as approximately 32.5% automotive-grade urea and 67.5% water and explains its role in reducing NOx through the SCR system.

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