SPN 4364: Often appears with SPN 4339, indicating SCR system issues

What is Fault Code SPN 4364?

Fault Code SPN 4364 indicates a problem with the Selective Catalyst Reduction (SCR) system on diesel vehicles. Specifically, it points to the SCR NOx conversion efficiency being lower than expected.

Urgency

If this code appears, the vehicle may enter a 25% derate mode to limit emissions. It's best to diagnose the issue promptly to avoid reduced performance and potential damage to the aftertreatment system. Driving should be minimized until the root cause is identified and repaired.

Parts Involved

The main components related to SPN 4364 are:

• SCR Catalyst: Converts NOx into harmless nitrogen and water using Diesel Exhaust Fluid (DEF)
• NOx Sensors: Measure NOx levels before and after the SCR catalyst to determine conversion efficiency
• DEF Doser: Injects DEF into the exhaust stream upstream of the SCR catalyst
• DEF Tank, Lines, and Pump: Store and deliver DEF to the doser

Common Causes

Potential issues that can trigger SPN 4364 include:

1. Degraded or poisoned SCR catalyst
2. Faulty NOx sensor(s)
3. Clogged or malfunctioning DEF doser
4. Contaminated, low-quality, or empty DEF fluid
5. Leaks or restrictions in the DEF delivery system

Diagnosis

To troubleshoot SPN 4364, follow this procedure:

1. Scan for all fault codes. Related codes like P20EE or P207F can provide clues.

2. Check DEF level and quality. Drain and replace if contaminated.

3. Inspect DEF tank, lines, and doser for leaks, blockages, or damage. Repair as needed.

4. Verify NOx sensor operation using live data. Replace if readings are out of range.

5. Monitor SCR efficiency during a test drive. If low, suspect a bad catalyst.

6. As a last resort, remove and inspect the SCR catalyst. Clean or replace if necessary.

Repair Costs

Depending on the root cause, fixing an SPN 4364 issue can range from inexpensive to quite costly:

• DEF fluid refill: $20-80 USD / €18-70 EUR
• NOx sensor: $300-800 USD / €260-700 EUR
• DEF doser: $500-1500 USD / €440-1300 EUR
• SCR catalyst: $2000-4000 USD / €1750-3500 EUR

Prevention

To avoid SCR issues that can set SPN 4364:

• Maintain proper DEF levels using high-quality fluid
• Address other emission system faults promptly to prevent catalyst damage
• Allow highway-speed driving regularly to ensure passive SCR regeneration
• Follow manufacturer's recommendations for aftertreatment maintenance intervals

By recognizing the symptoms of SPN 4364 early and systematically diagnosing the problem, repair costs and downtime can be minimized. Proper SCR maintenance is key to keeping the aftertreatment system healthy and preventing repeat faults.

Frequently Asked Questions About SPN 4364

What does SPN 4364 indicate?

SPN 4364 is a generic fault code that indicates the Selective Catalyst Reduction (SCR) system’s NOx conversion efficiency is lower than expected. The SCR system uses Diesel Exhaust Fluid (DEF) to convert harmful NOx emissions into harmless nitrogen and water. When conversion efficiency drops, it means the SCR system is not functioning optimally.

What are the symptoms of SPN 4364?

The most common symptoms of SPN 4364 are:

• Illuminated check engine light
• Reduced engine power (derate)
• Increased DEF consumption
• Ammonia odor from exhaust
• In some cases, there may be no noticeable symptoms other than the stored fault code.

What are the common causes of SPN 4364?

Some of the most common causes of low NOx conversion efficiency include:

• Degraded or contaminated DEF fluid
• Malfunctioning DEF injector (doser valve)
• Clogged DEF line, filter, or tank
• Faulty NOx sensors
• Damaged or poisoned SCR catalyst
• Exhaust leaks before the SCR
• Engine mechanical issues causing incomplete combustion

Can I continue driving with an SPN 4364 fault?

It depends on the severity. In most cases, the engine will go into a derated mode, limiting speed and power. This is to encourage you to diagnose and repair the issue promptly. Continuing to drive with an SCR system problem can lead to further damage and higher repair costs. It’s best to address the issue as soon as possible.

How do you diagnose an SPN 4364 fault?

Proper diagnosis involves:

1. Scanning for all fault codes to see if other components are implicated
2. Checking DEF level and quality
3. Inspecting the DEF tank, lines, and doser for clogging or damage
4. Testing NOx sensor operation and wiring
5. Monitoring SCR efficiency with a scan tool
6. Inspecting the SCR catalyst for signs of damage or contamination
7. Checking for exhaust leaks

Can low quality DEF cause SPN 4364?

Yes, using contaminated, expired, or diluted DEF is a common cause of low NOx conversion efficiency. The SCR system is very sensitive to fluid quality. Only use DEF meeting ISO 22241 specifications from a reputable supplier.

How do you test DEF quality?

The concentration of urea in DEF should be 32.5%. You can check this with a handheld DEF refractometer. Compare the refractive index of a sample from the tank to a reference chart. If concentration is low, drain the tank and refill with known good DEF.

What is a NOx sensor and what does it do?

There are typically two NOx sensors in an SCR system—one before the SCR catalyst and one after. They measure the concentration of NOx in the exhaust gas. The readings are used by the engine control module to determine how much DEF to inject and to calculate conversion efficiency. A faulty sensor can cause inaccurate DEF dosing and set an SPN 4364 fault.

How do you test a NOx sensor?

NOx sensors can be tested with a diagnostic scan tool. Compare the voltage readings of the sensors to manufacturer specifications. Readings that are slow to respond, erratic, or outside the acceptable range indicate a bad sensor. A physical inspection can also reveal damage to the sensor or wiring.

What is a DEF doser and what are the symptoms of failure?

The DEF doser, also known as the injector, sprays a fine mist of DEF into the exhaust stream. It is controlled by the engine module. Symptoms of a bad doser include:

• Crystallized DEF around the doser nozzle
• Excessive DEF consumption
• Poor NOx conversion efficiency
• Ammonia smell from exhaust
• Drivability issues like rough running or low power

Can a bad EGR system cause SPN 4364?

Yes, a malfunctioning Exhaust Gas Recirculation (EGR) system can cause incomplete combustion, resulting in excess soot and hydrocarbons in the exhaust. These contaminants can poison the SCR catalyst over time, reducing its efficiency. Always address EGR faults promptly.

Can a clogged DPF cause SPN 4364?

Indirectly, yes. A clogged Diesel Particulate Filter (DPF) can cause exhaust backpressure issues and heat damage to components like NOx sensors. Excess soot passing through a failed DPF can also contaminate the SCR catalyst. Proper DPF maintenance is important for overall aftertreatment health.

What are the possible repairs for SPN 4364?

Repairs can include:

• Draining and replacing contaminated DEF fluid
• Cleaning or replacing clogged DEF injectors, lines, or filters
• Replacing faulty NOx sensors
• Cleaning or replacing the SCR catalyst
• Repairing exhaust leaks
• Addressing root causes like EGR, DPF, or engine mechanical issues

How much does it cost to repair an SCR system?

Costs can vary widely depending on the specific issue and labor rates. Some example costs:

• DEF fluid flush and refill: $200-400
• NOx sensor replacement: $500-1200 each
• DEF injector replacement: $800-1500
• SCR catalyst replacement: $3000-6000+

Proper diagnosis is key to avoiding unnecessary repairs.

Can you clean an SCR catalyst?

In some cases, an SCR catalyst can be cleaned if contamination is not too severe. This involves removing the catalyst and using high pressure water or air to remove deposits. However, cleaning is not always effective and replacement is often necessary once the catalyst is poisoned.

How often should DEF filters be changed?

Maintenance intervals for DEF filters vary by manufacturer and application. In general, DEF filters should be replaced every 200,000 miles or 6,500 hours. However, it’s best to refer to your specific maintenance schedule. Premature clogging can indicate other issues like contaminated fluid or doser problems.

Is it safe to use aftermarket NOx sensors?

Factory NOx sensors are calibrated to very tight tolerances. While less expensive aftermarket sensors are available, they may not provide the same accuracy and durability. Using non-OEM sensors can lead to SCR system issues and faults. It’s best to stick with sensors from the dealer or a reputable supplier.

How can you prevent SCR system failures?

Some tips to avoid SCR issues:

• Always use high-quality DEF fluid from a trusted source
• Follow the manufacturer’s maintenance schedule for filter changes
• Address fault codes promptly to prevent cascading failures
• Allow highway-speed driving regularly to ensure proper SCR temperatures
• Avoid excessive idling which can cause DEF crystallization
• Use engine block heaters in cold climates to aid DEF thawing

What should you do if you suspect SCR tampering?

Tampering with or disabling SCR systems is illegal and can result in large fines. If you suspect an SCR system has been modified, it’s best to return it to stock condition as soon as possible.

This may require assistance from the dealer or an emissions specialist. Restoring proper function can be expensive but is necessary to avoid penalties and ensure clean operation.

Where can I find more information on SPN 4364?

For more detailed information on troubleshooting SPN 4364 and other SCR system faults, consult the following resources:

• Manufacturer’s service information and troubleshooting guides
• Online repair databases like Mitchell 1 or AllData
• Heavy-duty diesel forums like TheDieselGarage.com or TheDieselStop.com
• Technical training materials from organizations like the Technology & Maintenance Council (TMC)

Always follow the manufacturer’s recommended procedures and use proper safety precautions when working on emission control systems.