Hobbies Cars & Motorcycles Diagnosing the Great EGR Error Code Share PINTEREST Email Print This guy can figure out your trouble code issues, but you should try first!. Getty Cars & Motorcycles Cars How Tos Buying & Selling Basics Reviews Tools & Products Classic Cars Exotic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars SUVs Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. our editorial process Matthew Wright Updated January 10, 2019 Error Code P0401, which shows up in cars like the Chevrolet Volt, has become infamous for alerting mechanics to a poorly functioning exhaust gas recirculation (EGR) system. It's never good news, but there's always hope, especially if you're willing to try to diagnose the problem yourself before heading to your mechanic. A Good Valve Gone Bad Your car is running poorly. You tried to ignore it, but there's just no turning your back on the fact that your Check Engine Light (CEL) is staring you in the face, threatening to fail your car at its next emissions inspection appointment. You take your car to the repair shop for some professional analysis, and they tell you that you are going to need a new EGR valve. You're in the middle of imagining hundreds of dollars flying out of your wallet when you decide to ask if a new value is really necessary. Sure, the technician scanned the error code, but just because it means your EGR is failing, doesn't mean it needs to be replaced. Unfortunately, sometimes shop owners force their technicians to take the simplest, most obvious route rather than spending extra hours diagnosing a problem more thoroughly. In the shop's defense, replacing your EGR valve probably would eliminate the error code, turn off the CEL, and allow you to pass emissions inspection. What the P0401 Error Code Means One thing is certain: the P0401 error code specifically means you have "reduced flow" in your exhaust gas recirculation system. A stuck and/or ruined EGR valve will certainly cause reduced flow, but clogging anywhere along the EGR route can cause the same reduction—and there are plenty of points at which that can happen. Luckily, in most cases, all you need to do is clean these components to restore the system to its full capacity, which will turn off the check engine light and keep your money in your wallet. Clean or Replace? It's not difficult to clean your valve, although it is time-consuming. Remember to let your engine cool thoroughly before doing any service like this. The exhaust gasses are very hot and it can take some time for your engine components to cool down after heating up to full operating temperature. Then disconnect all electrical connections to your EGR valve and push all wiring safely out of the way. Finally, make sure you are wearing old clothing, have removed all jewelry, pulled back your hair if it's long, and are wearing eye protection for when you start straying carb cleaner. Safety first! Once you remove the EGR valve itself, shake it. If you hear the valve inside opening and closing, this means that it's still working and that you can probably get away with cleaning to get things back in order. If you don't hear this opening and closing sound, there is less of a chance that it will recover. But it won't hurt to give it a good clean anyway. You'll know once you drive the car if the EGR is beyond repair. Cleaning the rest of the system is simply a matter of removing any black stuff that has accumulated in the plumbing of the system. You can do this with carb cleaner. If you can take some of the hoses out and soak them, this helps. If not, just give them a good soaking inside with the carb cleaner, then use a soft wire brush, pipe cleaner, or small rag to wipe it through. Go for a Drive Once you give the system a serious cleaning, reassemble it and start driving. You'll know within a day or so whether you are still having a clogged EGR problem. If it turns out you still need to replace the EGR valve, go ahead and take it to your mechanic. But there's at least a 50 percent chance you've solved your problem without spending any cash at all.