P2BAC Code: NOx Exceedance – Deactivation of EGR

Did your OBD2 scanner register a P2BAC code? Here’s what it means, its usual causes and symptoms, as well as the best way to approach the issue.

What Does the P2BAC Code Mean?

The diagnostic trouble code (DTC) P2BAC indicates “NOx Exceedance – Deactivation of EGR.” Since the ECM can monitor NOx as a rationality check, the ECM will typically set this code if the level of NOx indicates the EGR isn’t working properly. But the ECM can’t determine what the issue is from NOx measurement alone. Typically other codes will be present along with the P2BAC code and should be addressed first.

Note About NOx

NOx is toxic, so the exhaust gas recirculation (EGR) system was designed to mostly prevent it from forming in the first place, although even on gas burners, a catalytic converter is mounted near the engine to handle the remaining NOx that forms despite EGR.

On modern diesel engines, there is a selective catalyst reduction (SCR) converter that takes care of NOx, but on diesels, a compound called diesel exhaust fluid must be injected for the SCR catalyst on these diesels to work.

To satisfy various OBD requirements, selective catalyst reduction systems will typically use a NOx sensor downstream of the SCR catalyst. If excessive NOx or ammonia concentrations exist downstream of the SCR, an OBD malfunction will be triggered, as many NOx sensors are sensitive to both NOx and ammonia (from the DEF).

Depending on the SCR control strategy, another NOx sensor may be installed in front of the SCR catalytic converter to determine the conversion rate of the SCR catalyst.

You can learn more about NOx in our technical discussion about the EGR system.

Note: The definition of code P2BAC can differ according to the vehicle manufacturer. Check the appropriate repair manual or repair database for the exact code definition.

What are the Common Causes of the P2BAC Code?

• Critical engine performance codes are set

What are the Common Symptoms of P2BAC Code?

• Illuminated check engine light
• Additional engine performance problems related to the other stored DTCs

How to Diagnose the P2BAC Code

The P2BAC code is a generic powertrain code that applies to various makes and models. Since it can have several causes and can come with other codes, troubleshooting it yourself is not the best option. It’s best to leave the job to professionals to make sure the problem is diagnosed and fixed properly.

How to Fix the P2BAC Code

For DIYers with advanced technical knowledge and hands-on experience who prefer to test and replace their vehicle’s EGR on their own, it will be helpful to rely on the appropriate repair manual or refer to an online repair database.