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Old 06-07-2014, 02:16 AM   #1
Focus-dude
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New CHT & CEL -- HELP

I just bought a 2004 Ford Focus SE 2.3L and recently been changing fluids and doing tune ups to get it back in tip top shape. Unfortunately, I am worried my optimistic tune-ups may have opened a can of worms. Let me explain.

I bought a new Cylinder Head Temperature (CHT) sensor thinking it would be an identical OEM part and fit. I come to find out the part was changed and also requires a different electrical connector. Wanting to put the new part on the car... I jumped right in and cut/solder the new electrical connector and put the new (stubby style) CHT in the car.

Two days later my CEL comes on and I check the code: P0128.

I know this code means the engine is not getting hot enough fast enough (seems like a blessing since I live in Southern California... but anyways...). I have read through the forums and most people state that this code is usually thrown when the thermostat is partially/fully open and thus the radiator is activated and keeps the car from getting hot.

BUT, I have also read that the ECU may need to be updated and for $40-$100 bucks the issue can be resolved for good. I have a hard time believing the new CHT and the P0128 CEL are not somehow related... wouldn't this mean my thermostat is fine?

What are your guys thoughts/opinions on this matter? ECU software need to be updated to match the new CHT part & sensor OR thermostat most likely stuck open?


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Old 06-08-2014, 01:52 AM   #2
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Originally Posted by Focus-dude View Post
ECU software need to be updated to match the new CHT part & sensor OR thermostat most likely stuck open?
Neither. Your thermostat worked before (correct? Car reached normal operating temp?) so concentrate on what changed: your cht sensor and wiring. Possibly your wiring is incorrect. I'm not sure why you'd change a working cht as part of a tune up. Is there anything preventing you from re-using the original part?

P0128 Thermostat malfunction
Possible causes:
- Leaking or stuck open thermostat
- Low engine coolant level
- Insufficient warm up time
- Faulty engine coolant temperature sensor
- Engine coolant temperature sensor harness is open or shorted
- Engine coolant temperature sensor circuit poor electrical connection
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Old 06-08-2014, 02:52 AM   #3
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Hey BC_ZX3,

I'm new to the car and on my previous cars the temperature sensors would constantly adjust depending on exactly what the engine was doing.

When I saw the temp needle in the Focus hop straight to vertical, even on cold starts, I figured the temp sensor was bad. I read a thread that said the CHT sensors can often go bad... wish I remember which thread it was.. but I figured the CHT activity was abnormal and went ahead and changed it. Unfortunately I changed it mid Wednesday.. and trash day is Thursday so the OEM part is long gone (lesson learned...the hard way).


I called the ford dealer today and the service-tech guy told me he has never heard of the ECU needing to be updated in the 2004 Focus cars..he said that is usually reserved for the latest cars which are filled with electrical components and customer-tech luxuries. I figured he is probably right and the ECU is fine.

I put the car into "Test Mode" and the new CHT sensor seems to be working just fine. On cold starts the needle stays at the bottom and gradually over 5 minutes or so creeps to the vertical position and stays there. The odometer area in test mode also shows the temp gradually increasing nicely up to and then plateauing at ~80C or 170-180F. I'm not exactly sure what temperature the thermostat is suppose to open up at, I think I read 192F somewhere. The previous owner seemed clueless about cars but regularly went to shops to keep everything in working order (I doubt the thermostat has ever been replaced).

With all that being said, I believe the CHT sensor is working as it should. I erased the codes and it didn't pop up today on me. I figure I'll just wait and see. If P0128 keeps re-appearing then I'll remove my current solder and see if somehow the connection was bad (I'm pretty sure I did a good job soldering it though).

It's kind of funny how Ford decided to normalize the temp sensor.. I should have read up on the problem more before I went in and changed it.

Anyhow, If the code comes back within the next day,week, or months I'll make sure to re-solder the connectors and re-post my findings. I know I'm not the first person who has gotten a CEL after installing the new stubby style CHT sensor.

Thanks for the feedback BC_ZX3.

Last edited by Focus-dude; 06-08-2014 at 02:57 AM. Reason: Grammatical
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Old 06-08-2014, 11:06 AM   #4
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Unfortunately I changed it mid Wednesday.. and trash day is Thursday so the OEM part is long gone (lesson learned...the hard way).
lol. Done this before. The balancing act between keeping too much junk and something you just maybe, possibly, perhaps, kinda might need in the future. Of course, as soon as you pitch it, it's the one critical part you need.

Glad to hear you seem to have it resolved.
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Old 10-11-2014, 09:36 PM   #5
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Figured I'd pass along a link to the thread I just update on this very problem. Had thermo & CHT changed multiple times but kept getting the P0128 problem. The factory ford thermostat is only a 180*f unit so it just didnt let my car get up to the 175*F required temp in time before triggering the CEL code (at least the Cylinder head area where the sensor is didnt get up to the right temp in time). I finally had to make a 188*F thermostat to solve the problem (after trying dozens of other things over the last 9 months)

Here was my fix: http://www.focusfanatics.com/forum/s...9&postcount=24
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