Still getting P0125 code after replacing thermostat

[morgant]

Shortly after I had evap work & cat replaced for inspection of my '01 Corolla last year, my check engine light came back on. I got the code read and it was P0125 and I was told I need a new thermostat because it was stuck open. It was about a year ago so I'm a bit fuzzy, but if I recall correctly, I wasn't really getting symptoms of running cool or not producing enough heat. I replaced thermostat and coolant (didn't do a full flush) and the check engine light came back on after driving it a little bit. Again, I haven't experienced any running cool or lack of heat output over the last year (and it always runs mid-point on the temp guage), so I didn't worry about it.

I should've gotten the code read again during the summer so I wouldn't be doing this work in the cold for inspection this year, but I'm lazy and I didn't. The garage again said code P0125, must be a stuck thermostat, and I was like "I just replaced it a year ago and I'm not getting any symptoms." So, at this point I have the following troubleshooting questions:

1) Should I replace the temp sensor next?

2) I also have read on a couple forums (thanks to someone here mentioning it in another thermostat thread; glad I did that research!) that Toyota P0125 can also mean primary O2 sensor, but that it's not officially documented anywhere so garages rarely catch it. Is that true? If so, should I replace that in addition to the temp sensor or instead? Since this car burns a ton of oil, a primary O2 sensor certainly seems like it could be needed.

3) If I need to replace the thermostat again, what's the correct one (temperature & with/without jiggle valve)?

4) I ran regular Prestone 50/50 (green) coolant because the coolant that was in there was green. However my garage suggests (as do some google search results) that I run the Toyota coolant as there an acid/alkaline difference. Obviously the latter is quite a bit more expensive so should I really switch to Toyota (red/pink)? If so, how best to flush the old & replace with Toyota?

On a related note, I've got a ton of other work to get this car to pass inspection, so anywhere I can save money but not waste time would be much appreciated. On the sensors, are there less expensive brands than OEM that are worth trying? Esp. if I end up doing temp, primary O2, and coolant (and possibly thermostat, but I personally doubt that at this point).

Thanks for any thoughts & suggestions! You all have always helped me out in a pinch.

[CelicaGuy13]

Could very well be the ECT sensor. Before replacing it, try to have someone monitor the temp readings using a scanner.

[morgant]

Yeah, I really should test the sensor. I don't know how I've gotten along so long without a multimeter, but I guess I'll go pick one up tomorrow to test the sensor with.

I can do the resistance test and if it happens to pass the test then that means it's not coolant/thermostat/thermometer and has something to do with O2 sensor or its heater. If it doesn't pass, then I'll need to test the coolant temperature to see if it reaches 180°F at operating temperature. Can I just buy a thermometer, verify it's accuracy in boiling water (212°F), and run the engine from cold w/the thermometer in the top of the radiator?

Looking in my Haynes manual, it actually does list code P0125 as "insufficient coolant temperature for closed loop" or "oxygen sensor heater malfunction" even though ODBC II code listings don't have that latter. So, while I understand the garage not thinking beyond thermostat, the least they could've done after reading the code and my explanation about having already replaced the thermostat would've been to test the ECT (temp) sensor.

Thanks for the suggestion and getting me thinking about further testing prior to replacements!

[CelicaGuy13]

No problem!

The coolant in your engine should NEVER boil. It's the antifreeze job and pressure of the coolant system that raises the boiling point.

Also. You could replace your radiator cap as well. If the cap is not holding the proper pressure. Then the coolant system won't operate properly. It's a cheap part to try since I'm sure you don't have a radiator cap tester kit. Lol also if you have an obd2 scanner that does "live data". Then you can just monitor the signal without probing the wires. Although nothing beats a good ole fashion multimeter check

There are other things that prevent coolant from reaching normal temp quickly. Like air in the system. Or a bad head gasket. But just because you changed the thermostat. Doesn't mean it cannot be that again. If you didn't get an OEM Toyota part, then the life expectancy of a part is unknown

[morgant]

When I initially changed the thermostat the check engine light came back on after only a short drive. So either the replacement was faulty out of the box or I've got other issues.

Good point about the radiator cap being a possibility. I didn't mean that I expected the coolant to boil, just asking if I'm correct that I can verify the coolant actually reaches 180°F while running (I would first test whatever thermometer I found using regular boiling water to make sure it's accurate).

As for air in the system, I thought I had properly "burped" it originally and I had checked it frequently and topped off coolant for a few weeks afterward. The coolant level hadn't changed since (almost a year now).

Unfortunately, I don't have access to any scanner, let alone one that can read "live data". i've got a multimeter on order, so I'll start the testing & ruling out possibilities once it arrives.

Thanks again!

[greyrolla]

+1 on Toyota Red

[leakyseals]

if your sure the electronic fans are operating properly, thermostat was installed correctly....

There can be a host of problems associated with having the green coolant in there. Even worse if the red is mixed with green. Toyota's Red is designed for compatibility with Toyota's aluminum head/block/radiator and the sensors used. wrong coolant, wrong mix you can get corrosion, false readings, could be anything.

If it were me I'd get the green out of there asap to rule that out. Go to Toyota and get the red concentrate, mix 50/50 with distilled water so you end up with 2 gallons. The only real way to get it all red and not run the risk of gelling stated above is a flush. But I have gotten away with doing the radiator drain as follows:
Do the drain from the radiator as you did before, adding the 1st gallon. drive for a couple days/week, do it again with the remaining. Should get most of it. If you see gelling in the overflow over time, do it again. Its really not that costly considering the options.

[morgant]

Last I checked the electric fans were operating normally, but I can double check. I'm assuming you're thinking they're running too frequently and over-cooling, considering the error code.

I can definitely switch to the red coolant, but won't do so until after I've troubleshot sensors and such (it'd be a waste to have to drain it if I have to replace the thermostat or ECT sensor). I had brought up the coolant type issue with my garage and they agreed I should switch it back to red in the near future, but that they doubted that'd cause any real issues. That said, I've owned the car since December 2008 and last year's thermostat swap was the first I'd checked the radiator and it was already running w/green, so that's what I replaced with. So, it certainly could've corroded sensors by this point, if that's what'd happen by running green.

A full flush can be done as follows, right?

1. Starting with the engine cool, drain the radiator.
2. Fill radiator with water.
3. Run engine & heater with radiator cap open until engine gets to operating temperature and starts circulating coolant.
4. Drain & fill with water again & run for a few mins.
5. Then repeat with new coolant (probably twice to get the mixture right).

Thanks for the further suggestions!

[leakyseals]

The distilled water is important, dont use tap for the mix. The minerals/scale in tap water can cause problems down the road. That being said, with your flush method tap water is really the only way and should work. A mall amount of tap will be left after this, using distilled with the actual mix should keep that issue minimal. Aquiring a cheap test kit can help balance things after. There may be another way to dump most of the water/coolant. Going to an area I don't know for sure on your car. A cooling system has 2 hoses. One low on the block where the water pump draws, the return we see on top. Removing that draw hose underneath surely will drain most of the fluid/water, much more that the radiator drain because now the block can drain. But thats going to make a mess so be prepared. Also be vigilant not to let things get too hot during the water flush. Good luck.

[morgant]

Thanks again! I'll be doing all this troubleshooting & coolant flush next weekend, so I'll report back with how it goes.