DEF "contaminated"?

Tahoe threw a code yesterday when heading out with 1 segment of the DEF level gauge highlighted (1/8 tank?) and got a DEF contaminated (can't remember if that was the exact word) error and an admonition we'd be held to 65 MPH in 100 miles. Well, I stopped at the one Costco in the area without Peak and walked away empty handed. When I restarted the truck, no code and ran 150 miles with no more issues (picking up a jug of Peak at another Costco along the way). It's been sitting quite a bit the past few months, so I'm guessing the DEF got a little crystallization and once it sloshed around a bit, everything got mixed back up and reabsorbed, making the sensor happier.

Anyone else see this?

While anything is possible, I have a seasonal vehicle that has had the same DEF in it for two years. No DEF related issues. It's highly unlikely that it crystallized in anything less than a year's time. It could have been getting low and maybe left the sensor exposed or something like that. Don't get hung up on Peak DEF. Go to Walmart and buy it (Super Tech) anywhere for less than nine bucks. It's all the same stuff, been running it for 20 years.

As soon as you get the first warning at roughly 1000 miles, go buy a 2.5, dump it in and keep motoring....

Yeah, I've used mostly the cheap stuff, from Walmart and Menard's, but Peak is only $13 or $14 at Costco. Doesn't seem if DEF shows on the level indicator the quality sensor should have an issue.

GM uses a quality sensor in the DEF tank. I'm not exactly sure of the type - there are various methods for measuring the quality of the DEF. The ideal concentration is 32.5% Urea - the rest is distilled water. If the quality sensor gets below a threshold it's going to give you a code telling you the DEF is "contaminated."

There's a PID on my Banks iDash that shows the DEF concentration. I've noticed that after driving for awhile it increases about 1-2% concentration. Point being, sloshing around in the tank is good for the DEF.

You mentioned it sat for for the past few months. Has it been hot? Does it sit outside? DEF does not like extreme temperatures.

Personally I keep my tank as full as possible. Once I drop to 50% I add another jug. 1) It keeps the tank full and reduces condensation in the tank and 2) it introduces fresh DEF into the tank on a more frequent basis than waiting to nearly empty the tank. I'm putting in fresh DEF every ~6 months versus waiting closer to a year to refill it. This is based on how many miles we drive - everyone's situation is different.

I have had the same message as you on the speed will be reduced after X distance but no mention of DEF contamination. What I realized though is the dealer clearly didn’t fill my DEF tank before I took delivery and is why my brand new Yukon XL was so “thirsty” for DEF fluid during the first couple thousand miles. Filled it to the brim 2 months ago and it’s only moved one bar on the DEF gauge since. Haven’t driven a ton but definitely have close to 1,000 miles since then. Seems more like it.

Same thing happened with mine. 5 miles on the odometer when I drove it off the lot. DEF gauge was “full” and within the first 500 miles I think I dropped 3 chiclets on the gauge. Seemed like way too fast of DEF consumption.

Since then usage is way more reasonable, as in it takes ~4K miles to drop to 50% after I fill it.

JayceeP said:

I have had the same message as you on the speed will be reduced after X distance but no mention of DEF contamination. What I realized though is the dealer clearly didn’t fill my DEF tank before I took delivery and is why my brand new Yukon XL was so “thirsty” for DEF fluid during the first couple thousand miles. Filled it to the brim 2 months ago and it’s only moved one bar on the DEF gauge since. Haven’t driven a ton but definitely have close to 1,000 miles since then. Seems more like it.

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This is usually the issue, the gage is inaccurate at best, and you may be lower than you think on DEF. I usually don't like filling mine clear to the brim, as if you keep it around a half when it gets low you are putting in a 100% fresh DEF and not being diluted by the remaining half a tank that is still in there. Keeps it optimum I think....

StephenPT said:

There's a PID on my Banks iDash that shows the DEF concentration. I've noticed that after driving for awhile it increases about 1-2% concentration. Point being, sloshing around in the tank is good for the DEF.

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Ah, good to know, I'll set that PID to watch it!