P0402 is an OBDII trouble code that’ll pop up in the GMC Terrain. It has to do with the emissions system and stands for:
P0402: EGR → Excessive Flow Diagnosis
The EGR system on a vehicle is responsible for recirculating exhaust gasses in order to lower the vehicles emissions output. The amount of gas cycled back into the engine is typically not more than 10%. These gases help keep the engine’s combustion temperature under 2500 degrees Fahrenheit.
P0402 is a general code, which means that regardless of the year (1996+) and the model, it’ll have the same meaning regardless of which manufacturer built the vehicle (GMC Terrain or not).
Related: P0420 GMC Terrain
GMC Terrain P0402 Symptoms
Most of the time there are not going to be symptoms when P0402 is triggered, other than the service engine soon light itself. In some extreme cases, you may notice a slight ping or knock.
- Service Engine Soon Light– P0402 will illuminate the service engine soon light.
- Loss in Power– In more extreme cases, you may notice some adverse effects on the engine. These would include a low engine power, hesitation, and dying at idle.
- Knocking Sound– In certain cases, there may be a noticeable knock that comes from the vehicle. It’ll sound almost like piston slap.
P0402 Causes: GMC Terrain
Diagnosing what is causing the P0402 code in the Terrain is typically not very tricky. Here are the most common causes:
- Sensor Issues– There are many sensors that the vehicles computer relies on to accurately operate the EGR system. Some of these sensors include the EVP, DPFE, and MAP sensors. When one of them goes bad, they can cause P0402.
- EGR Valve Itself is Bad– The EGR valve itself is often the cause of P0402. They typically aren’t that expensive
- Vacuum Issues– Some EGR valve relies on engine vacuum to operate. Others rely on an electric signal. If there is not enough vacuum present to allow for the proper operation, the EGR may not close. P0402 will then be thrown.
- Sensor Issue– The Differential Pressure Feedback- EGR sensor can go faulty and cause the EGR valve to stop operating properly and throw the code.
Terrain P0402 Fix
The good news with P0402 is that it rarely ever costs much money to fix, and it is usually pretty easy to get the problem, since it is right on top of the engine. Most folks see P0402 Insufficient EGR flow, look up the price of an EGR valve, see how cheap it is and just throw a new one on.
That can be a mistake. That’s because the EGR valve is not always going to be the culprit. Here are some of the most common fixes to help clear the P0402:
- Check the EGR Tube- It’s a good idea to check the tube going to the EGR because it’s often the cause of P0402, easy to do, fast, and replacing it is very affordable. If it’s cracked, you’ll need to get a new one. If it’s clogged you can get away with cleaning it and replacing it.
- Check DPFE Sensor Voltage- If you are good with a voltage meter go ahead and test it. Here’s a good article on how to do that from It Still Runs: How to Test DPFE Sensor.
- Test the EGR Valve- A vacuum gauge will help you determine if the EGR valve itself has gone bad. Here’s an article from Autozone on how to test it with a vacuum gauge.
Conclusion
Testing your EGR system is relatively easy to do. The most challenging aspect is the DPFE sensor. If you aren’t sure you can tackle that, you can always test everything else first. If there is anything you would like to add about P0402, please feel free to add a comment below. Good luck!!