How to Clear Service DEF System Message on Duramax Engines

When the Service DEF light comes on in your dashboard, it usually means that the DEF (Diesel Exhaust Fluid) levels have dropped below the required level. If you don’t attend to it, the vehicle will gradually lose speed and hamper the engine performance.

I will show you step by step how to clear the service DEF system message on Duramax engines.

How To Clear Service DEF System Message On Duramax Engines?

Diesel exhaust fluid doesn’t directly influence engine performance. It doesn’t increase the efficiency of diesel by any means. But it makes the emission more environment friendly by reducing nitrogen oxides from the exhaust smoke.

So, the Environmental Protection Agency (EPA) ma`1de the use of DEF mandatory for post-2010 diesel engines.

When the DEF levels run low, your engine will slowly start to limit the speed. You can expect to drive 50 to 100 miles with the Service DEF System light on. At one point, the speed will be restricted to 4 miles per hour.

As you can imagine, it will be a nightmare if you are far away from home and need to drive a long distance before you can refill the DEF tank.

The obvious way to resolve this issue is to fill up your DEF tank. But, as we pointed out, that might not be possible if you are far away from a fuel station, truck stop, or auto parts store. If you find yourself in such a jam, what should you do? If your vehicle has a Duramax engine underneath it’s hood, read on to find out how you can bypass DEF restrictions.

That said, we must remind you that you should refill the DEF tank as soon as you can. We don’t recommend putting it off unless you absolutely have to. Driving on low DEF levels can damage the emission control system of your engine. The service DEF light might also indicate some problems other than a near-empty DEF tank.

DEF System Message On Duramax Engines: Potential Causes & Solutions

Poor DEF Quality

Usually, you should get a ‘’Poor DEF Quality’’ message when the DEF quality in your tank gets compromised. But, in some cases, you might also see the Service DEF notification pop up for this reason. Ideally, diesel exhaust fuel is a mixture of 32.5% urea and 67.5% deionized water. A different mixture ratio or the presence of impurities can reduce the effectiveness of the DEF.

If you have sufficient fluid in your DEF tank, but the Service DEF message is still showing, you should test the diesel emission fluid. You can buy DEF contamination test kits online and in auto parts stores. Collect the sample of diesel emission fluid from the tank and dip the strips in it. Then, compare the strip color with the reference chart.

If the fluid turns out to be contaminated, you should replace it immediately. Sometimes the DEF we buy could be stored in suboptimal conditions. You must store diesel fluid at 50°F or less temperature to maximize its shelf life. If the fluid checks out okay, you would need to consider other potential underlying issues.

Faulty DEF Injector

A malfunctioning DEF injector can also trigger the Service DEF light. Disconnect the fuel injector from the engine and inspect it thoroughly. If you see crystallization and sediment buildup, you should clean it and reinstall it. Also, check the injector for leaks and damages before you put it back. 

If you don’t see any visible obstacle or damage, it would be best to conduct a Reductant Injection Quantity Test. Connect a diagnostic tool to your truck to start the test. Disconnect the DEF injector from its port and keep a container underneath it to collect the DEF. The test will keep the DEF injector operational for two straight minutes. 

So, you will get to see the spray pattern of the injector. If you notice any irregularities there, there might be a problem with the injector. If the injector manages to spray out the fluid at an efficient rate, the container should have about 40 and 55 milliliters of DEF after the test finishes. If it has less, you should consider changing the DEF injector. 

DEF Level Sensor Malfunction

The DEF level sensor measures the level of fluid inside the DEF tank and notifies the system when the fluid level plummets below recommended levels. So, a malfunctioning DEF level sensor could erroneously trigger the vehicle’s ECU, even when the tank is full. In some cases, cleaning the sensor is all you need to do to fix this problem. 

The residual buildup can compromise the performance of the DEF level sensor over time. So, you need to clean it from time to time. However, if your truck shows voltage codes or communication codes, cleaning the DEF level sensor won’t make your problem go away. You must replace it to restore proper DEF sensor functionality in your vehicle.

How To Bypass The Service DEF Restriction Without Fixing The Problem

Step 1. Switch Off The Engine

Park the car, turn off the engine, and take the key out of the ignition before you begin. To be extra safe, you can disconnect the negative battery terminal, but it should not be necessary. Keeping the engine shut should do.

Step 2. Locate The Speed Sensor

The speed sensor measures the rotation rate of the crankshaft. It can read how fast the crankshaft is spinning and deduce the speed of the vehicle. When the service DEF system restrictions kick in, the speed sensor makes sure that the vehicle doesn’t go faster than the capped speed limit. 

You have to disconnect the speed sensor to cheat the system. In Duramax engines, you should find the road speed sensors on the transfer case. If you are still having trouble finding it, you should look it up on the internet to get a precise location. 

Step 3. Disconnect The Sensor

Carefully disconnect the sensor from the wiring harness of the engine. Undo any bolt or clip that attaches the sensor to the engine. You would need a socket wrench and a set of pliers to get the job done. Keep the sensor in a secure place so that you can easily find it later. 

Step 4. Start The Vehicle

Now get into your vehicle and start the engine. The service DEF system notification might still show, but you can now drive above the speed limit. If you can’t change gears, move to neutral, and then try shifting gears. You should be able to do it now. 

Final Words

How to clear the service DEF system message on a Duramax engine? After you go through our article, you might find it easier than you had previously thought. But, we would like to remind you that bypassing the service DEF system restrictions should only be considered when you need to bail yourself out of an emergency situation.

Knowing how to trick your car’s ECU into believing you have taken care of the DEF problem without actually doing it can be a lifesaving trick if you find yourself stranded in the middle of nowhere. But as soon as you get the chance, you need to check the car, detect the problem and take necessary maintenance measures. Don’t forget to reinstall the speed sensor afterward. 


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