A check engine light can be frustrating, but this alert doesn't always indicate a severe problem with your vehicle. Check engine lights (CELs) appear when your car's computer detects a problem with one or more sensors and stores a diagnostic trouble code (DTC). DTCs log symptoms, although they can often help a shop uncover the root cause of an automotive problem. 

The exhaust gas recirculation (EGR) system is one potential source of DTCs. The EGR system is a crucial part of your car's emissions equipment. The EGR valve, the primary component of the EGR system, returns a small portion of your car's exhaust into the combustion chamber to help reduce emissions. If your computer detects a problem with the EGR valve, it will trigger a DTC.

Which Codes Related to the EGR System?

EGR system issues can trigger numerous DTCs, although the most common one will typically be P0401. This generic code indicates an issue with the amount of exhaust gases entering the combustion chamber. Your car's computer controls the EGR valve and, as a result, knows how much gas should pass through it at any given time. Insufficient exhaust gas flow will trigger code P0401.

Note that you may also receive other DTCs related to the EGR system. These codes typically follow the P040x format. Other codes can provide some additional information for vehicles with multiple EGR circuits. P0404 (exhaust gas recirculation circuit range/performance) may also indicate a problem with the EGR sensor.

Are EGR Codes Critical Problems?

Most EGR codes aren't critical issues, but they can affect your vehicle's performance and prevent you from passing state-required emissions or smog tests. Once an EGR issue triggers one or more DTCs, the problem is severe enough that the balance of gases entering your car's combustion chamber will likely be off. As a result, you might notice poor idling or hesitation when accelerating.

A faulty EGR system can also cause misfires, which can be potentially damaging to your engine. Misfires typically trigger additional DTCs, and your check engine light may start flashing. If you notice these symptoms, avoid driving your car more than necessary. In general, you shouldn't do more with a misfiring car than drive somewhere safe and call for a tow.

What Causes EGR Codes?

Since the EGR system is relatively straightforward, the two most likely culprits for an EGR malfunction are the EGR valve or EGR sensors. Depending on your make and model, you may have one sensor to detect exhaust gas flow and another to detect the position of the EGR valve. A faulty valve will affect how much gas passes through the system, while a faulty sensor can produce unpredictable symptoms.

The DTCs stored in your computer can help point a technician in the right direction, but they'll rarely point to the faulty part. Instead, an experienced shop will use these codes as a starting point and test the EGR valve and sensors to determine the underlying problem. This thorough approach will ensure you can find and repair the problem without unnecessarily wasting money on extra parts.

Contact an auto repair shop to learn more.