Doug_B_928

I've had a check engine light happen 3 times in the past two weeks. Each time when I read the codes it showed the error message in the title. I clear the fault but within a couple of days it comes back. Prior to this beginning, maybe a week before, my wife had a flat and I had to put a winter wheel/tire on the left rear and limp it home so I could change all of the tires to the winters while awaiting a replacement tire (the flat was not patchable). The winter wheel/tire is a slightly different diameter and so I just drove it this way for a few miles to get it home. The error message did not begin right after this but it's the only out-of-the-ordinary circumstance so I thought I should mention it. The truck runs great.

Has anyone ever seen this fault code and know what it means?

Doug_B_928

So I had seen some posts about error codes and a loose gas or oil cap. They weren't associated with this fault code but thought I'd check. The gas cap was not turned on completely. Haven't had the code since, knock on wood. I thought I should post this update for anyone that gets this error code and does a search in the future.

snappy

The only sporadic error codes i have seen, and i have seen allot of them
Was doe to a bad battery. Not saying that this is your problem, but i had sporadic error codes ranging from PCM to airbag ect pup up and go away a day later to resurface again within a week.

Changing out the battery did the trick for me.

If you have a or can borrow a "Battery Reconditioner" I would try that first to see if it helps.
It cured the problem for over a month but at least i then had some idea that it might be the battery

Doug_B_928

Good point. The volts appear good but I understand that a battery should always be a suspect with odd codes. The light returned this am so I stopped by the dealer and they said that P8121 is usually a bad thermostat. Will see if the extended warranty covers it.

hahnmgh63

That is correct in that a P8121 is a Cooling system Thermostat code. Always check your coolant level first and top off with the correct Coolant or Distilled water at the least. What does your temp gauge read? Also, if it comes down to it and you do the Thermostat yourself or at an independent shop remember there is a TSB for new sealing rings for the on the intake socket of the coolant regulator housing. But as mentioned above a weak battery can give all kinds of quirky issues too.

Doug_B_928

Coolant level was good (I checked that after reading elsewhere that it might be coolant related). Temp is always the same; about 180F. But, I haven't looked at it with my durametric, which should show the actual temperature. Apparently the gauges are set so that they won't fluctuate unless there is a significant change in the temperature.

hahnmgh63

You are correct about the gauges. The signal for the gauge goes through the ECU and it takes a fairly good temp change to move the gauge. Maybe just a quirk but definitely check it with your Durametric to be safe. Good luck and let us know the outcome.

User 4221

The code indicates the thermostat is stuck open. The engine doesn't warm up to a certain temp in an allotted time and throws the code. It is for emissions so the engine heats up to operating temp quickly.

I get this one on my GTS and just clear them.

Doug_B_928

I guess it picked a good time to get stuck open, since it's not heating but cooling that is the concern at this time of year. I'm was planning to pick-up my trailer from storage this weekend, about 120 miles of driving with the trailer and car in the trailer (my 914 project that will take me forever to finish). Since the thermostat is stuck open, there shouldn't be any issues with pulling the extra load, right? It's not a must so I can wait until it is fixed if that would be better.

Doug_B_928

Ended up not making the trip for the trailer to be safe. Thermostat was replaced at by the dealer this week. It was stuck open. BTW, the last time I checked the code it had changed from 8121 to 2181 with an additional description of something like "cooling system performance."

touareg

I know, 3 years ago, was it ever resolved?

Doug_B_928

Yes. I should have been more clear in how I said it. The code changed as mentioned above the last time I read codes prior to having the thermostat replaced. After the thermostat was replaced the code did not return.

stronbl

Be careful with implausible signal errors. They can be representative of the true problem or they can be a red herring. Sometimes an implausible signal is a sympathetic error code resulting from a "daisy chain" of another error code. They may or may not be real valid error codes so it is always best to try to figure if this is the case or not. It can be very time consuming and frustrating. But in the case for this thread it sounds as though there was a clear indication of a cooling system problem, so that helped eliminate other possibilities.

When my NOx sensor failed on my CD, I received an implausible signal for other emissions related parts that were just fine. Likewise when my turbo boost hose cracked and I lost turbo boost and reduced engine performance, I received an implausible signal for the diesel particulate filter regeneration system. Once the boost hose was repaired the regeneration issue went away as the engine now was back to full power and the regeneration could occur automatically as I drove.

touareg

Thanks guys, yeah I am getting an implausible signal from the coolant sensor, and once in awhile the gauge goes to cold. Was infrequent, now increasing in frequency.

Trying to see if it is a wiring issue, bad sensor, or a coolant leak....