AutoCodes – Generic – P21B3

P21B3 Code - Closed Loop NOx Absorber At Limit Too High

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 Mass Air Flow (MAF) Sensor value not learned, EGR valve sticking or restricted, Thermostat outlet restricted. 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: P21B3 indicates Closed Loop NOx Absorber At Limit Too High.
Common causes: Common causes include Mass Air Flow (MAF) Sensor value not learned, EGR valve sticking or restricted, Thermostat outlet restricted.
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 832

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 P21B3 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 if the time duration after an active soot regeneration is requested exceeds a calibrated threshold and the Diesel Oxidation Catalyst inlet temperature has not reached the light-off temperature threshold to begin closed-loop control.

OBDII Code P21B3 - Closed Loop NOx Absorber At Limit Too High - AutoCodes.com — P21B3 Code - Closed Loop NOx Absorber At Limit Too High

What does this mean?

What are the Possible Causes of the DTC P21B3?

Mass Air Flow (MAF) Sensor value not learned
EGR valve sticking or restricted
Thermostat outlet restricted
Faulty EGR cooler bypass control valve
Faulty EGR airflow control valve
Leaking or damaged NOx absorber catalyst

How to Fix the DTC P21B3?

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 P21B3 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 P21B3 code?

Need more information on how to fix the P21B3 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P21B3 code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P21B3 Dodge — Closed Loop NOx Absorber At Limit Too High
P21B3 RAM — Closed Loop NOx Absorber At Limit Too High

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 P21B3 Code

1. What does the OBDII code P21B3 mean?

P21B3 indicates that the vehicle's closed-loop NOx absorber system has detected a limit that is too high, meaning the system is unable to properly reduce nitrogen oxide emissions.

2. What are the common symptoms of the P21B3 code?

Symptoms may include an illuminated check engine light, reduced engine performance, increased fuel consumption, and elevated NOx emissions.

3. What causes the P21B3 code to trigger?

Common causes include a malfunctioning NOx sensor, a failing NOx absorber or catalyst, exhaust leaks, or issues with the engine control module (ECM).

4. Is it safe to drive with a P21B3 code?

Driving with a P21B3 code is not recommended as it can lead to higher emissions, potential damage to the NOx absorber system, and reduced engine efficiency.

5. How is the P21B3 code diagnosed?

A technician will use an OBDII scanner to confirm the code, inspect the NOx sensors and absorber system, check for exhaust leaks, and test the ECM for errors.

6. Can a faulty NOx sensor cause the P21B3 code?

Yes, a faulty or damaged NOx sensor can give incorrect readings, leading to the P21B3 code being set.

7. How do you fix the P21B3 code?

Repairs may involve replacing a faulty NOx sensor, repairing exhaust leaks, replacing a degraded NOx absorber, or reprogramming/updating the ECM.

8. Will clearing the P21B3 code solve the issue?

Clearing the code without addressing the underlying issue will only temporarily remove the warning. The code is likely to return if the problem persists.

9. How much does it cost to repair the P21B3 code?

The cost depends on the root cause. Replacing a NOx sensor may cost $200â€“$500, while repairing or replacing the NOx absorber can be significantly more expensive.

10. Can a dirty or clogged NOx absorber cause the P21B3 code?

Yes, a clogged or degraded NOx absorber may prevent proper absorption of nitrogen oxides, triggering the code. Regular maintenance can help prevent this issue.