SPN 520373 – Aftertreatment Outlet NOx Sensor

What is Fault Code SPN 520372?

Fault code SPN 520372 FMI 16 indicates "Selective Catalyst Reduction (SCR) Closed Loop Control at Maximum Limit". This means there is an issue with the SCR system that is causing it to use excessive diesel exhaust fluid (DEF) in an attempt to reduce NOx emissions.

Urgency and Severity

If this fault code appears, do not continue driving the vehicle. The SCR system is a critical emissions control component. Ignoring this fault will eventually lead to DEF inducement, causing the check engine light to activate and the vehicle to derate (limit speed to 5 mph). Take the vehicle to a qualified repair shop or dealer as soon as possible to diagnose and repair the underlying issue.

Applicability

This is a generic OBD-II code that applies to many makes and models of diesel vehicles equipped with SCR systems, including:

• Freightliner Cascadia
• Detroit Diesel DD13, DD15, DD16 engines
• Volvo trucks
• Mack trucks
• Mercedes-Benz vehicles

Key Components Involved

The main components of the SCR system that can trigger this fault code include:

1. DEF tank and delivery system – Stores and supplies DEF to the dosing unit
2. DEF dosing unit – Injects DEF into the exhaust stream before the SCR catalyst
3. SCR catalyst – Converts NOx into harmless nitrogen and water using DEF
4. NOx sensors – Measure NOx levels before and after the SCR catalyst to determine conversion efficiency

Common Causes

Some of the most common causes of fault code SPN 520372 include:

• Contaminated or degraded DEF fluid (not 32.5% urea concentration)
• Faulty DEF dosing unit injecting too much fluid
• Clogged DEF injector or lines
• Failing NOx sensors providing incorrect readings
• Damaged or clogged SCR catalyst
• Leaks in the DEF delivery system

Diagnosis Steps

To properly diagnose the root cause of SPN 520372, follow these steps:

1. Use a scan tool to check for all active fault codes. Address any other codes before proceeding.
2. Check DEF quality and concentration using a refractometer. It should be 32.5% urea. If incorrect, drain and flush the DEF tank.
3. Perform a DEF quantity test to check the dosing unit. The dispensed amount should be 108-132 ml. Replace the dosing unit if failed.
4. Visually inspect the DEF injector, lines, and connectors for clogs or damage. Clean or replace as needed.
5. Run a forced DPF regen and SCR efficiency test. Monitor NOx conversion efficiency, which should be over 90%.
6. If efficiency is low, perform a NOx sensor test. Replace sensors if they fail the test.

Repair Solutions

Once the faulty component is identified, repair or replace it as follows:

• Drain and refill contaminated DEF fluid with fresh 32.5% DEF
• Replace faulty DEF dosing unit if it fails quantity test or is physically damaged
• Clean clogged DEF injector and lines, replace if cleaning is ineffective
• Replace failing NOx sensors based on test results
• Replace damaged SCR catalyst if dented, cracked or melted
• Repair any leaks found in DEF tank, pump, lines or connectors

After repairs, clear fault codes and retest the system to verify the repair.

Associated Fault Codes

Other fault codes that commonly appear alongside SPN 520372 include:

• SPN 3226 – DEF tank level circuit
• SPN 3031 – DEF dosing unit
• SPN 3216 – DEF pump
• SPN 4094 – NOx sensor

Prevention

To prevent SCR issues and fault code SPN 520372 from reoccurring:

• Only use high-quality DEF fluid that meets ISO 22241 specifications
• Regularly inspect the DEF tank, lines, and connectors for leaks or damage
• Replace the DEF filter every 150,000 miles or when clogged
• Service the SCR catalyst every 200,000 miles to remove ash buildup
• Address any fault codes promptly to avoid further damage

Repair Cost

The cost to repair issues causing fault code SPN 520372 varies depending on the component:

• DEF fluid flush & refill: $100-300 USD / €90-270 EUR
• DEF dosing unit replacement: $800-1500 USD / €720-1350 EUR
• NOx sensor replacement: $300-600 USD each / €270-540 EUR each
• SCR catalyst replacement: $1500-3000 USD / €1350-2700 EUR

Note: Prices are estimates and may vary based on vehicle make, model, and repair shop.

By understanding fault code SPN 520372, its causes, diagnosis, and repair, vehicles can be properly serviced to restore the SCR system and maintain emissions compliance. Always refer to the manufacturer's service information for specific troubleshooting procedures.

Frequently Asked Questions About SPN 520372

What does fault code SPN 520372 mean?

SPN 520372 FMI 16 indicates "Selective Catalyst Reduction (SCR) Closed Loop Control at Maximum Limit". This means there is an issue with the SCR system that is causing it to use excessive diesel exhaust fluid (DEF) in an attempt to reduce NOx emissions.

How urgent is it to address SPN 520372?

If this fault code appears, do not continue driving the vehicle. The SCR system is critical for emissions control. Ignoring the fault will lead to DEF inducement, causing the check engine light to activate and the vehicle to derate (limit speed to 5 mph). Take the vehicle to a qualified repair shop or dealer as soon as possible.

What vehicles can get fault code SPN 520372?

SPN 520372 is a generic OBD-II code that can appear on many diesel vehicles with SCR systems, including Freightliner Cascadias, trucks with Detroit Diesel DD13/DD15/DD16 engines, Volvos, Macks, and Mercedes-Benz.

What are the key components involved with SPN 520372?

The main SCR system components that can trigger SPN 520372 are:

1. DEF tank and delivery system
2. DEF dosing unit
3. SCR catalyst
4. NOx sensors

What are common causes of SPN 520372?

Common causes include:

• Contaminated or degraded DEF fluid
• Faulty DEF dosing unit injecting too much fluid
• Clogged DEF injector or lines
• Failing NOx sensors
• Damaged or clogged SCR catalyst
• Leaks in the DEF system

How do you diagnose the root cause of SPN 520372?

Diagnosis steps:

1. Check for other fault codes and address those first
2. Test DEF quality and concentration (should be 32.5% urea)
3. Perform a DEF quantity test on the dosing unit (should dispense 108-132 ml)
4. Inspect the DEF injector, lines and connectors for clogs/damage
5. Run a forced DPF regen and SCR efficiency test
6. Test NOx sensors if efficiency is low

How do you repair issues causing SPN 520372?

Repair solutions:

• Drain and refill contaminated DEF fluid
• Replace faulty DEF dosing unit
• Clean clogged DEF injector and lines
• Replace failing NOx sensors
• Replace damaged SCR catalyst
• Repair any DEF system leaks

What other fault codes commonly appear with SPN 520372?

Related fault codes include:

• SPN 3226 (DEF tank level)
• SPN 3031 (DEF dosing unit)
• SPN 3216 (DEF pump)
• SPN 4094 (NOx sensor)

How can you prevent SPN 520372 from reoccurring?

Prevention tips:

• Only use high-quality DEF fluid meeting ISO 22241 specs
• Regularly inspect the DEF system for leaks or damage
• Replace the DEF filter every 150,000 miles
• Service the SCR catalyst every 200,000 miles
• Address fault codes promptly to avoid further damage

What is the typical cost to repair SPN 520372 issues?

Repair costs vary by component:

• DEF fluid flush & refill: $100-300
• DEF dosing unit: $800-1500
• NOx sensor: $300-600 each
• SCR catalyst: $1500-3000

Can you clear SPN 520372 by disconnecting the battery?

No, simply disconnecting the battery or clearing fault codes will not resolve the underlying issue. You must diagnose and repair the root cause, then clear the codes afterwards.

Is SPN 520372 covered under warranty?

It depends on the vehicle's age, mileage, and warranty terms. Emissions components like the SCR system are often covered by an extended emissions warranty (e.g. 5 years / 100,000 miles). Check your vehicle's specific warranty for coverage details.

Can bad DEF fluid cause SPN 520372?

Yes, using contaminated, degraded, or incorrect DEF fluid (not 32.5% urea concentration) can cause the SCR system to use too much fluid, triggering fault code SPN 520372.

Will SPN 520372 clear after replacing a faulty component?

Not immediately. After repairing the faulty part, you must clear the fault codes, then run a forced DPF regen and SCR efficiency test to verify the repair. The fault will only clear if NOx conversion efficiency passes the test.

How often should you check DEF fluid quality?

Test DEF quality with a refractometer at every oil change interval or if you suspect contaminated fluid. DEF should be crystal clear with 32.5% urea concentration.

Can a failed NOx sensor cause SPN 520372?

Yes, a failing NOx sensor can provide incorrect readings to the SCR system, causing it to dose too much DEF and set fault code SPN 520372. Always test NOx sensors when diagnosing SCR efficiency issues.

How do you test DEF injector quantity?

To test the DEF dosing unit quantity:

1. Unbolt the dosing unit from the SCR
2. Run a DEF Quantity Test with a diagnostic tool
3. Measure the dispensed fluid (should be 108-132 ml)
4. Replace the dosing unit if the quantity is out of spec

What is a normal NOx conversion efficiency?

A healthy SCR system should have over 90% NOx conversion efficiency. Lower efficiency indicates an issue with the SCR components, DEF fluid, or sensors.

Can you drive with SPN 520372 active?

No, do not drive a vehicle with SPN 520372 active. The SCR system is in closed loop at maximum limit, which will soon lead to DEF inducement, check engine light, and 5 mph derate. Only drive the vehicle to a repair shop for diagnosis.

What happens if you ignore SPN 520372?

Ignoring this fault code will cause the SCR system to use excessive DEF in an attempt to reach target NOx conversion efficiency. This will quickly lead to DEF inducement, illuminating the check engine light and limiting vehicle speed to 5 mph until the fault is repaired.

By understanding fault code SPN 520372, its causes, diagnosis, and repair, you can effectively troubleshoot SCR system issues and prevent costly downtime. Always consult the manufacturer's service information for specific diagnostic and repair procedures for your vehicle.