[TheWolf]

Howdy. Last night I was driving and my check engine light came on. I went to an auto store today and they did the computer thing and they said it was Code P0128 and they said it should go off by itself after a few driving cycles.

What do you think? Everything seems to be running fine. No engine noises, no sputtering, AC runs fine, etc.

 

[sailor]

You can find a listing of many codes here:http://www.focusfanatics.com/forum/showthread.php?t=59286 (from the How To stickies at the top of the page)

That one, for "thermostat malfunction", can sometimes pop up on an unusually cold day for your area if the car doesn't warm up as fast as usual.

Something to keep an eye on, pat attention to heat avail. and the gauge reading for anything unusual - checking it out closer if it doesn't go away.


"Parachute Sale, Save on Factory Seconds"

 

[TheWolf]

I'm going to disconnect the car battery which will reboot the system (and turn off the check engine light). If the light comes back on, that means there's something wrong with the car.

 

[TheWolf]

Update: I disconnected the battery on March 1st (the day after the light came on) and when I reconnected the battery, the Check Engine Light was off and it remained off until tonight (March 17).

This means there is something wrong with the car and is likely the PO128 code that I got last time.

Looking online and what this code is, I see there are a number of things that could be wrong:

Defective Engine Thermostat
Defective Engine Coolant Temperature Sensor
Defective Intake Air Temperature Sensor
Defective Cooling System
Low Engine Coolant
Dirty Engine Coolant, causing incorrect Coolant Temperature Sensor readings
Defective, always running Engine Cooling fan(s)


Where can I go to find out exactly what's wrong?

 

[amc49]

You can't, most codes have way more than one cause and you have to work that out or pay others to do the same. In your case the thermostat needs to be changed (sticking open) is most likely cause but no 100% guarantee there so don't go looking for it.

Looking for the quick cure? The last three items on the list can easily be checked by you and don't take two minutes to do. We can't breathe for you...............

FYI, when you let others check for dtcs look over their shoulders to get the EXACT numbers yourself, counter guys will often make up codes to sell you parts that do nothing. As well, bringing those numbers online here if not 100% correct can have us telling you to change hundreds of dollars worth of parts that again don't need changing.

 

[TheWolf]

Update on situation: Coolant levels are fine and not dirty, so that eliminates two of the problems. The engine is not over-heating so that eliminates a stuck-closed thermostat.

It's very likely it's just a thermostat stuck open.

 

[sailor]

Here's info. on a way to check for that:Ford Focus Forum, Ford Focus ST Forum, Ford Focus RS Forum - View Single Post - Low temperature heater output

The diagnostic mode will let you watch temps as it warms up from a cold start.

Unusually slow warm up/not getting fully warmed up is what your code implies, this is a way to check that. Gauge rapidly goes to the center area when temps approach normal, it's a fancy idiot light as it reports only cold, normal, very hot & too hot.

 

[TheWolf]

New update: Went to Advanced Auto Parts and they checked the code and showed me. Thermostat is stuck open so the engine is cooling.

 

[amc49]

There you go, one tracks the issue down by process of elimination and deductive thinking.

 

[TheWolf]

Check Engine light went off by itself while driving.

 

[amc49]

There is no reason why it can't, the stat can begin to work again correctly. Here's what I'd say though, once a stat begins to stick, it can stick shut as well as open, and you are now risking the engine and entire car by simply leaving it be. Could be looking at a severe overheat from sticking stat in the future, wouldn't it be plumb silly to lose a car over that? Last time I checked stats were still cheaper than cars.......................

 

[TheWolf]

There is no reason why it can't, the stat can begin to work again correctly. Here's what I'd say though, once a stat begins to stick, it can stick shut as well as open, and you are now risking the engine and entire car by simply leaving it be. Could be looking at a severe overheat from sticking stat in the future, wouldn't it be plump silly to lose a car over that? Last time I checked stats were still cheaper than cars.......................

True. My dad told me to go see a mechanic to get the issue resolved.

 

[Somethin2]

Hi,

Was wondering if you ended up replacing Thermostat on your car? As I have a 2006 was curious if it was difficult to do or not? If yes what instructions did you USE to complete this process? I have a PO128 which is with engine light on. 2 Different scanners Autozone and Napa. Both showing same code over a month apart before each diagnostics done. No loss of coolant no overheating.

Thanks,

Somethin2

 

[amc49]

'...go see a mechanic to get the issue resolved.'

 

[Leora83]

YouTube is great for stuff like this. Just search for your year and model and "thermostat replacement" and it'll give you a bunch of videos that walk you through the process. This one is easy for a DIYer.

If your thermostat is sticking OPEN you will have no heat inside the car (not good in January!), and eventually the car will go into limp mode because it can't generate enough heat to run properly. If it's sticking CLOSED you will overheat and blow hoses/caps/pipes...not a good way to do a coolant flush.

It's a cheap easy fix. You've had two warnings that there's an issue...time to fix it. Either check out some videos and do it yourself, or take it to a garage and have it done.

 

[Slimester]

I had that code and it was a sticking thermostat. Replaced it myself while watching a couple of youtube videos. In one video the guy is removing a lot of stuff to get access to it. In the other video the guy replaced the thermostat without having to remove anything but a couple of connectors. That's how I did mine and it was pretty easy.

 

[Stoenr]

I got the code now twice on my 07 2.0 ZX3

Last month came on, plugged in this $15 OBDII adapter that pairs with most Android devices, fired up my tablet, pulled the code, checked coolant temp, was about 160-165f if memory serves me, and this is what it normally runs.
Cleared it, nothing for a month until last night same code came back.

So I decided to Data log it, and temps were normal, engine warmed up as intended, and I had plenty of heat. Drove it about 55 miles, highway and some city. Stopped it went down to about 150f, moving stayed a constant 160-165, one time got on it to merge it went to 173 but quickly settle back to 165. I think it ran a tad warmer in the summer, but not by much.

Both times it was cold. This time it was really cold, like 10f out! As you can see intake temp was 12.2 lol.

So, a freezing cold fluke? or sensor on the fritz? Not to worried as I cannot see any mechanical issue or running issue. And not about to work on this in these temps!

 

[Stoenr]

Light came on again. /looks for electrical tape

 

[Stoenr]

Getting annoyed now

Ok, seriously the light is back for its 5th time. Everything is working as intended. Today it happened and it was 31 degrees out, every other time is was like 10 degrees, so I brushed it off.
Possible bad sensor? I don't think Ford even makes them any more as the PN I found showed discontinued.

Any advise?

 

[Slimester]

Yeah, you need a new thermostat.