exhaust gas temp 3 fault??

Depends on the actual code. Most common codes are for circuits out of range, not temps too high. You could be right, It could be simply too hot.

 

would a pyrometer confirm this? Iwas thinking of getting one anyway

 

Yes ........

 

Pyrometer shows exhaust heat right after the turbo. Exhaust temp #3 sensor is the last sensor in the after treatment system. If the exhaust was too hot out of the engine you would also have over temp codes for exhaust temp 1 and 2 as well. If temp#3 is reading right and it is too hot that means the regen burn is happening in DPF, not the DOC. During a proper regen, DOC inlet temp, or temp #1, should be about 550-600. DOC outlet temp, or temp #2, should be 1000-1200 degrees. DPF outlet temp should be 950 to 1200. In a healthy system, the engine injects fuel into the exhaust during a regen that the DOC burns to create the heat. The heat is used to BAKE down the collected soot in the DPF. When something goes wrong and the DOC is not properly burning, unburnt fuel or other combustible contamination such as engine oil collect in the DPF and starts to BURN. This is somewhat uncontrolled and temps can get really high. When the ECM sees #3 temp exceed 1250 degrees it will log code 1972. The ECM turns on the check engine light and regens are disabled until the next key cycle. If the temps exceed 1472 degrees, the red STOP engine comes on, regen and egr operation is disabled. With these faults, the exhaust temps up to the DOC will be normal, so installing a Pyro would be worthless to diagnose this problem.

 

my scan gaudge shows code 3245....which i cant find anyware.

Thanks for the thorough explanation though. This helps me understand the problem. So, if its not showing code 1975 (unverified) it should just be a bad sensor?

 

I find SPN 3245 which is "suspect parameter number". 3245 is for the #3 exhaust temp sensor. There are about 5 or 6 different faults though. The SPN is usually accompanied by a FMI number, which is "failure mode indicator", or basically how it is faulty. The ECM can detect the sensor circuit shorted, open, temp too high or temp not changing at all. So you are still down to either a sensor problem or a problem with the wiring. You could try a new sensor, but they are typically seized in and require a torch to get out.

 

The only reason the a temp fault 3 light would appear alone is if the filter was doing an uncontrolled regeneration (such as raw fuel burning in the filter). The other high temp lights would come on first if it was a true uncontrolled regen. Since it is coming on at startup, it shouldn't be hot.
Please don't use fuel additives or power cleaner on any truck fitted with a DPF. It causes extra ash that reduces the time between filter cleanings.

 

I have the same issue, 2009 Freightliner Cascadia DD series 60, it occurs when I am coasting down hill or using the engine break down a hill. All three warning lights come on and in a matter of seconds the engines shuts down before I can pull off the road. It has happened three times this week. Dealer said exhaust temp high, but no one has fixed it yet...any advice here

 

were taking about a cummins with regen, a detroit is a totally different animal. None of us had motor shut down either.

 

I got a 2008 Kenworth with the Paccar engine in it. I got the code SPN 3245.

From what HeavyD was saying it sounds like I got fuel burning in my DPF. About 5000 miles ago my truck busted an injector which then dumped a ton of fuel into the exhaust. I would think that all that fuel would of burned off by now. Do you think that is what caused the problem? If so, how can I fix it? Do I need to have the DPF cleaned?