Help diagnosing P0125 code

I'm recently getting a P0125 check engine code, "Insufficient Coolant Temperature for Closed Loop Fuel Control".

Using Hybrid Assistant I've plotted vehicle temps below, note outdoor temp 30F. Peak engine coolant temp was 194F. It does not seem to be overheating.

Also it does seem like it's getting hot enough to enter closed loop control, Hybrid Assistant reporting S4 state.

When I turn on the cabin heater, coolant temp dips a little. Seems fine and normal.

I believe the Gen 3 has two coolant temp sensors, one in the block (ECT sensor), one in the tubing tee above that.

Can anyone tell me which of those two sensors is the likely culprit?

Also, the thermostat seems to be working, right?

Thanks for the help.

Posted via the PriusChat mobile app.

 

Thanks, @ChapmanF . I was just going to update my post with your helpful explanation from over in this thread.

Is it possible this could be caused by starting the car in this temperature, and immediately putting the cabin heater on the "HI" setting? I know Prius treats this "HI" setting differently, but can't remember exactly how. In Gen2 i believe that turned on some resistance heaters, but not sure if the cooling loop is somehow affected as well...?

I've read elsewhere on PriusChat that the Prius' wax thermostats are very reliable. Mine is at 140,000 miles, so maybe due for one?

I'm trying to avoid throwing parts at the problem, thanks for your help!

 

That is good advice, i wish my past self had known! I did water pump at 110k...

Sounds like consensus is thermostat. i'll probably take another few Hybrid Assistant recordings and see if i can further confirm the diagnosis. Shade-tree wrenching is difficult when there's 2 feet of snow! Spring is coming though.

 

OK, I have confirmed both coolant temp sensors are working using an OBD2 sensor.

For others looking to do this, i used the android app "Car Scanner ELM OBD2", which was free, and shows both the 7C0 and 7C4 PID's (again linking this post). Both are showing similar values during engine warmup, which is consistent with other descriptions of their behavior. see the attached image below.

Guess i'm doing a thermostat!

Am i correct that this is not a major, immediate concern? Engine is not overheating, both sensors are working. Coolant level is fine, and almost new (~30k on it, tops).

I should be able to replace it within a week or two, but need to drive the car in the meantime. I don't want to risk a roadside breakdown and tow...

 

One other symptom, which i'm guessing is due to this suspected bad thermostat. If anyone can confirm this symptom is related, i'd appreciate that.

Prior to the MIL coming on, i had not been regularly using Hybrid Assistant. Using it recently though, i notice that the car is staying in S3 at times when i was expecting it to be fully transitioned to S4 (e.g. ~30 minute rural drive, then get on interstate, but car stays in S3).

I can't seem to find an exact meaning to the S1-S4 states, but would this "lingering" in S3 be explained by a failed / failing thermostat as well?

 

Yes, i stopped using the "HI" setting. No, the code has not come back yet. It was showing a permanent P0125, which was not clearable, but that is now gone as well. I didn't mention this in my original post, but had already cleared these codes one other time, but i did not change away from using the "HI" setting after that first reset, only after the second code clearing attempt, which so far if have no codes.

 

Still no recurrence of this code, weather has been pretty similar, and still no use of HI heat setting. I'll probably do the thermostat later anyway. I'm glad it's less urgent so i can do it with a few other things at the same time (EGR stuff, intake manifold, throttle body, PCV valve).

It really had not occurred to me this could just be a "bug" in the car's software / heating & cooling design. Thanks for the input everyone!

 

Well, yes, my reason is this is new behavior. I live in chilly weather, and this is this car's third winter. This is the first time i've seen this code. The HI heating position is used by another driver of the car, and i never thought anything of it before. It seems odd to me this is now happening unless something has degraded or changed.

Thinking through options with the thermostat, it seems like: 1) it is clearly not failed closed, no overheating. 2) it could be failed open, or 3) it could be sealing poorly and leaking.

• If the thermostat is not leaking, the block sensor will get hot quickly, then the exhaust heat recirc coolant sensor should start to see some temperature lagging the block. (From the other thread, looks like 7C4 may be the block sensor, which would be the one to get hot quicker).
• if the thermostat is leaking, the block sensor will get hotter faster, but the exhaust heat recirc coolant sensor will see that heat slowly build right away.
• if the thermostat is failed open, the block sensor and exhaust heat recirc coolant sensor will be almost in lock step from the beginning.

In fact i did this already, starting the car from cold (32F when i tested), see the attached plot. A couple observations:

• I'm not sure why the 7C4 sensor has a low limit of ~61F... the car sat overnight outdoors, at ~28-32F, so no way it's 61.
• The 7C0 sensor starts increasing in temp ~90 seconds after engine start.
• The 7C4 sensor starts increasing above the ~61 value after 140 seconds.

To me this pattern seems to match either a very leaky, or failed open thermostat. If the timing for this test is that the block sensor should be above something higher than 110F after 85 seconds, this is clearly not doing that.

Maybe these are the wrong PID values? Or let me know if you think my diagnostic isn't quite right too!

 

I wasn't very satisfied with the temperature graphs above, so explored the OBD app, and added a few more parameters, including engine RPM.

The car was run a few hours earlier, so the coolant temp was above ambient.

Some observations:

• 7E2 and7E0 seem to be the PID's of interest, likely in the block, as these increase first.
• 7C4, for whatever reason, starts at ~60F, but then matches 7E2 and 7E0 beyond this point.
• 7C0 lags all of the above, i suspect this is the exhaust heat recirc coolant sensor (let's say EHRCS)
• The fact that the EHRCS lags the block sensors by only ~20 seconds is interesting.

@ChapmanF could you post some further details on the P0125 code details? your earlier response indicated a check temperature, but not what the value was. thanks!