I Accidentally Put DEF in My Diesel Tank, Says GM Owner Who Faced A $15K Disaster

• A GMC Sierra owner accidentally filled his diesel tank with diesel exhaust fluid (DEF).
• DEF in a diesel tank can damage the fuel system, leading to hugely expensive repairs.
• An independent shop saved the owner thousands compared to the dealer’s repair quote.

We all make mistakes, but rarely do they have the potential to cost us upwards of $15,000. That’s the kind of error one owner of a 2023 GMC Sierra 1500 with the 3.0-liter Duramax Turbo Diesel accidentally made at a local gas station. Thankfully, a calm head prevailed, and he managed to avoid a bill that large.

Still, he wasn’t completely off the hook, as he ended up coughing up around $2,000 after filling his diesel tank with DEF.

What Exactly is DEF, and Why Should You Care?

Diesel exhaust fluid (DEF) is a required additive to newer diesel trucks. It works by neutralizing nitrogen oxide emissions from the combustion cycle. For years, the only way to put it into a truck was via a box full of fluid. Lately, some fuel stations have added a DEF pump next to diesel pumps. That’s where this story went sideways for our protagonist.

Read: Rivian Owner’s DIY Repair Saves Thousands After Mishap And Teaches Us A Lesson

The owner shared his story on Reddit under the handle hambone809 , explaining: “I was distracted, on the phone, not paying attention — and yeah, you guessed it: I started filling the DEF into the diesel tank .” On the surface, it doesn’t seem like a massive issue, but DEF can wreak havoc on a diesel fuel system. The fluid crystallizes, gumming up the works, which means fuel can’t get into the cylinder properly. That’s when the real trouble starts.

What did the owner do once he realized his mistake? A few simple reasonable steps. First, he didn’t turn the truck on. Even putting the truck into neutral would be problematic since it would prime the fuel pump. We’ll circle back to that.

The Parking Brake Dilemma

Second, he called a tow truck and ran into a new problem. Without power, the Sierra can’t go into neutral to get loaded up onto the tow rig. Evidently, this tow truck driver knew a trick. “There’s a small nipple on the transmission underneath the truck. He clamped some vice grips on it and rotated it to shift into neutral manually.”

That only worked for so long, though, because with the truck in neutral, the parking brake came on as an additional layer of safety. Here’s where the fuel pump comes into play again. To get the parking brake off, the owner had to power up the truck briefly to release it. Sure enough, the fuel pump primed and the owner got a warning on the dash. “Service Fuel System – Water in Fuel Line,” it said.

The Price of a Mistake

Worried about the impending repair bill, he called the dealer. “With 4 gallons of DEF in the tank, we’d have to replace the entire high- and low-pressure fuel system. That’s a $15,000 job,” they told him. Thankfully, an independent shop was more flexible. It offered to try to replace just the low-pressure fuel components and see what happened.

Sure enough, after that, bleeding the high-pressure components, and about $2,000, the truck is back on the road. Several commenters offered suggestions on how to avoid this should others be in the same situation.

For one thing, it’s worth wondering why the owner didn’t pull the fuel pump fuse before powering the truck on. Clearly, he was thinking about it, and removing the fuse could’ve rendered it completely incapable of delivering fuel. Second, some surmised that the tow truck driver could’ve gotten the truck on the rig even with the parking brake in place.

Lessons Learned

Hindsight is 20/20 and the owner had some additional thoughts to share with the community.

“Know where the manual shift nipple is under your truck — it saved me in the towing process. And if this ever happens to you: • Do not start the truck • Get it towed • Avoid the dealership if possible — independents may save you thousands,” he wrote. Consider this a tale of what happens when new technology goes from a positive to a negative.

In an old diesel truck, DEF isn’t in the equation. On top of that, parking brakes are manual, not electric. Progress is good, but sometimes it creates all new issues to sort out.