SPN 3031/FMI 10: DEF Tank Temperature Erratic Fault
This fault is all about the temperature reading coming from your DEF tank. Instead of showing a smooth, steady reading like it normally should, the value is bouncing around or jumping in ways that don’t make sense. When the truck’s computer notices this odd pattern, it stores this code to flag that something in the DEF tank’s temperature monitoring isn’t behaving correctly.
Step 1: Rule Out a Cooling System Problem First
Before touching the DEF tank at all, check whether there are any existing cooling system issues or complaints on the truck. The DEF tank’s temperature system is tied into engine coolant circuits, so a cooling problem elsewhere can easily throw off this reading. If there’s a cooling issue, fix that first — it may resolve this fault on its own. If the cooling system checks out fine, move on to the next step.
Learn How To Fix Battery Is Not Charging — Showing Low Voltage
Step 2: Run a Component Test
Connect your diagnostic tool and turn the ignition to ON without starting the engine. From here, run the built-in Output Component Test. Keep an eye out for whether fault SPN 3031 with FMI 3 or FMI 4 pops up while this test runs. Keep in mind this specific test can only be run once per key cycle, so make it count.
If either of those related faults show up, fix those first, clear the codes, confirm the repair worked, and the truck should be good to go. If neither shows up, continue on.
Step 3: Compare the Reading to Reality
Pull up the DEF Tank Temperature parameter on your diagnostic tool. Then take off the DEF tank filler cap and use a handheld thermometer, like an infrared temperature gun, to measure the actual liquid temperature inside the tank directly.
Compare the two numbers. If the reading on your diagnostic tool is more than 10°C (50°F) higher or lower than what you measured by hand, that confirms the sensor reading truly doesn’t match reality, and it’s time to dig into the hardware itself.
Step 4: Check the Tank Header Wiring
Unplug the connector for the DEF tank’s temperature and level harness. Take a close look at the pins inside — you’re checking for anything bent, spread apart, or corroded.
If you spot any pin damage, repair it and you’re likely done. If the pins look completely fine, the DEF tank header assembly itself has probably failed internally and needs to be replaced.
Step 5: Check the Coolant Valve Wiring Too
There’s a second connector worth checking — the one for the DEF tank header’s coolant valve. Disconnect it and inspect the pins the same way: looking for bent, spread, or corroded contacts.
Again, if there’s visible pin damage, a simple repair should sort it out. If the pins are in good shape, the tank header assembly is the likely culprit and will need to be swapped out.
Good to know: This part is officially logged under the component code “DEF Tank Header” for anyone tracking parts or writing up the repair.
Common Questions About This Fault
Can a bad DEF tank sensor cause the truck to lose power?
Yes, it can. Left unresolved, this fault can trigger a power derate of around 10 percent, so it’s worth getting checked before it reaches that point.
Is this an expensive repair?
It can be, depending on what’s actually wrong. If it’s just a wiring fix, the cost stays low. But if the sensor inside the DEF tank itself needs replacing, the cost goes up since that part and the labor involved cost more.
Can hot or cold weather trigger this fault by mistake?
No, weather isn’t the cause here. This fault points to an actual hardware problem, such as damaged wiring or a failed tank header, not outside temperature swings.
Does this affect the DEF fluid itself?
It can affect how accurately DEF gets dosed into the exhaust system. Since the system relies on an accurate temperature reading to manage dosing correctly, an erratic signal can throw that accuracy off.
How long does it take to replace the tank header?
Plan on more than 2 hours for this repair. It’s a more involved job than a simple connector fix, so it takes longer to complete properly.
