Bad DEF?

[Kjon]

Got a check engine light caused by ECM error code SPM 3521 FMI 11 (Bad DEF?). Engine was running normally and after about 200 miles the check engine light came on and after about 400 miles went out only to return an hour later. DEF is about 12 months old (Due to lack of travel during the Covid fiasco) and will soon be replaced with fresh DEF at the next fuel stop. Engine continues to run smoothly with no loss of power. 1) Has anyone experienced this? 2) Is there something that can be added to DEF to preserve / rejuvenate it?

[Lt Dan]

I doubt the DEF is bad, but you could use a refractometer to check the quality. That's one of the codes that comes up when the DEF head fails. I would contact Tiffin with that code to see what they say, but my bet is on a failed head. You should still be under the chassis warranty. The Shaw DEF heads have been known to fail, ours went out a little over a year ago.

[NMBluept]

As Lt. Dan mentions the 3521 code could be an indication of a failing DEF header or as I'm inclined to believe an actual indication of deteriorated DEF quality. Remember the DEF sensor looks at quantity, quality, & temp. With older DEF, especially if it has frozen and been exposed to high temps it could be of marginal quality. Since the error is intermittent it could be real in which case replacing or even topping off the DEF might clear it. When my DEF header failed there was no early indication, we went from all is well to low DEF, red CEL, and shut down now all with over 3/4 tank of fresh DEF. I made it to a Cummins shop where a DEF header failure was diagnosed. Cummins directed me to contact Tiffin where I was immediately directed to Freightliner - "that's a chassis issue". If you do have a DEF header failure it could be covered by the chassis warranty if still valid or most certainly by the Freightliner emissions control warranty ( 5 yrs/100,000 mi.). You may want to look into obtaining a DEF Sensor Simulator (DSS). Instructions on building a DSS can be found in these forums.

Safe travels,

[Kjon]

Discovered the DEF was 9 months old not 12. Fresh DEF was added Jan 1, 2023. We did have an awful freeze here in PA last December when the temp dipped to 4 degrees. The check engine light was on this AM when we started out but went off about 1 hour into our trip. I added about 7 gallons of fresh later when I fueled up. The light has not returned. Does anyone know how the DEF head knows if the DEF is not up to standard? I have a refractometer but have never used it. My chassis is a Powerglide and apparently one of the last RED's made on it as Tiffin switched to Freightliner.

[NMBluept]

The DEF sensor is a solid state device that uses a capacitance sensor for quantity and probably a resistance sensor for quality. DEF is 32.5% pure urea and 67.5% distilled water so it should have a very specific resistivity. I'm not 100% sure about this, but it is definitely a solid state/semiconductor device and like most semiconductors it either works or doesn't work. The caveat to that would be electrical connections which could cause intermittent operation.
When my DEF header failed the theory at the time was that the sensor enclosure failed allowing liquid to damage the semiconductor components.
I'm attaching a picture of the Freightliner DEF header for your viewing pleasure :-) A couple of things to note: the larger stainless line is loop for engine coolant for temp control, the white object is a strainer for the DEF pick-up, and larger black tubing is the sensor enclosure, the round black object is likely a standoff for the bottom of the tank.

Safe travels,

[thegats1]

I'm definitely not an expert on this. I built my DEF Sensor Simulator designed by some very smart individuals on this forum. Turned out that I needed it one day. Here are the diagnostic codes Cummins reported for my DEF header failure. I also show the dash lights that lit up when it happened. As suggested above, I called Tiffin and Ricky guided me through the process seamlessly.

[bsedor]

Originally Posted by Lt Dan

I doubt the DEF is bad, but you could use a refractometer to check the quality. That's one of the codes that comes up when the DEF head fails. I would contact Tiffin with that code to see what they say, but my bet is on a failed head. You should still be under the chassis warranty. The Shaw DEF heads have been known to fail, ours went out a little over a year ago.

I found a DEF quality tester at Rural King today. It looks like a battery tester where you siphon the fluid up like a turkey battery and see how many particles float in the glass tube. I have used it yet.