AutoCodes – Generic – P21B4

P21B4 Code - NOx Absorber Over Temperature

Quick Answer

When the engine runs in DeNOx mode, the Powertrain Control Module (PCM) monitors how much post-injection fuel has been injected. Common causes include Exhaust Temperature Sensors failed in-range low, Unintended fueling, Damaged NOx absorber catalyst. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P21B4 indicates NOx Absorber Over Temperature.
Common causes: Common causes include Exhaust Temperature Sensors failed in-range low, Unintended fueling, Damaged NOx absorber catalyst.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 1,021

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Aug 09, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P21B4 Description

When the engine runs in DeNOx mode, the Powertrain Control Module (PCM) monitors how much post-injection fuel has been injected. Once it exceeds a threshold, the measured O2 Sensor 1/1 is supposed to be lower than a threshold. If after a sufficient amount of post fuel is injected, the O2 Sensor 1/1 value is still high for a calibrated time, the fail counter is incremented by 1. If it is not, it is decremented by 1. If the counter has been incremented or decremented for a calibrated number of times, and the counter value is greater than a calibrated value, this fault is set. The Diagnostic Trouble Code (DTC) will set when the NAC outlet temperature exceeds a calibrated high threshold for a calibrated period of time.

OBDII Code P21B4 - NOx Absorber Over Temperature - AutoCodes.com — P21B4 Code - NOx Absorber Over Temperature

What does this mean?

What are the Possible Causes of the DTC P21B4?

Exhaust Temperature Sensors failed in-range low
Unintended fueling
Damaged NOx absorber catalyst

How to Fix the DTC P21B4?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P21B4 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P21B4 code?

Need more information on how to fix the P21B4 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P21B4 code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P21B4 Dodge — NOx Absorber Over Temperature
P21B4 RAM — NOx Absorber Over Temperature

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P21B4 Code

1. What does the OBDII code P21B4 mean?

P21B4 indicates that the NOx absorber sensor has detected an over-temperature condition in the NOx storage/reduction system of your vehicle.

2. What are the common symptoms of the P21B4 code?

Symptoms may include the check engine light turning on, reduced fuel efficiency, increased emissions, or the vehicle entering limp mode.

3. What causes the P21B4 code to appear?

Common causes include a malfunctioning NOx temperature sensor, a faulty exhaust gas recirculation (EGR) system, excessive engine heat, or exhaust leaks near the NOx absorber.

4. Can I continue driving with the P21B4 code?

Driving with this code is not recommended, as it may lead to increased emissions, potential damage to the NOx absorber, or further harm to the exhaust system.

5. How is the P21B4 code diagnosed?

A technician will typically use a scan tool to confirm the code, inspect the NOx sensor and related wiring, check for exhaust leaks, and test the temperature sensor's functionality.

6. What repairs can fix the P21B4 code?

Repairs may include replacing a faulty NOx temperature sensor, repairing exhaust leaks, addressing cooling system issues, or recalibrating the engine control module (ECM).

7. How much does it cost to fix the P21B4 code?

Costs vary depending on the root cause but generally range from $150 to $500, including parts and labor, with sensor replacement being a common repair.

8. Can a dirty or clogged NOx absorber cause the P21B4 code?

Yes, a clogged NOx absorber can cause overheating, triggering the P21B4 code.

9. How can I prevent the P21B4 code from recurring?

Regular maintenance of the exhaust system, timely replacement of sensors, and ensuring proper engine cooling can help prevent this code from returning.

10. Is the P21B4 code specific to certain car makes or models?

No, the P21B4 code can appear on any vehicle equipped with OBDII and a NOx absorber system, though it is more common in diesel vehicles.