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7th Gen In the years from 2001 to 2005 Honda released it's 7th Generation Civic. Chassis codes: EM2, ES1, EP3, EU1

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Old 01-08-2011   #1
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Temperature stays cold

engine heats up but temp gauge says cold, sometimes it floats up to about the 1/4 mark. OD is delayed and fuel mileage sucks, engine is hot. New thermos stat, and i just replaced the little temperature sensor above the TS housing, but problem persists. I believe i got codes po118 (high input on temp sensor...both sensors new and old same code) and coolant temp below TS regulating temp.

the only other sensor i see is the one below the TS housing, not sure if that's the problem. Im almost certain judging by how hot the heater is that the actual temperature of the coolant is fine, just not reading properly....

thanks.
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Old 01-08-2011   #2
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Re: Temperature stays cold

new honda thermostat?
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Old 01-08-2011   #3
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Re: Temperature stays cold

no I had another as well, also not a honda OEM, just installed it and problem persists. not even sure the make of the thermos stat it came from car-quest (know someone who works there get a good deal on em) The heat gauge goes up to normal for a bit, then it falls of right to the cold and the transmission falls out of overdrive (im guessing its supposed to do that when it registers cold) and it will stay cold for minutes, or an hour who knows. the heater stays hot while this happens so i'm sure its not the thermos stat.

I replaced the top sensor that looks like this:Click the image to open in full size.

wiring looks fine nothing frayed or broken.

i dunno its bugging me now!
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Old 01-08-2011   #4
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Re: Temperature stays cold

well ive had really bad luck with the non honda thermostats. they never work right. and from what you posted, you for sure have an actual problem with the coolant not getting hot enough. its not just a false reading. you can still get hot air even with cooling system problems, since the coolant can still circulate thru the heater core and does not have to be at operating temp to give hot air. there was no reason to change the ect sensor either, since they almost never fail (its a simple temperature sensitive resistor in there). first thing i would try is a new honda thermostat. after that we can go from there if it doesnt get fixed.
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Old 01-08-2011   #5
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Re: Temperature stays cold

the one in there now is a cheap thermos stat, but the previous one was an oem honda i got new last summer. It didnt fix the problem either, but i didn't care as much in the warm weather. My temperature laser gun thingy confirms the coolant lines top and bottom are at operating temperature and do not fluctuate with the temp guage on the dash.

Ive heard IAT can be bad, not sure what the name of the lower sensor is in the TS housing, but It seems suspicious.
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Old 01-08-2011   #6
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Re: Temperature stays cold

K my buddy had an OEM thermos stat, it did nothing. Still having this issue. Like i said, i replaced the small temp sensor on the thermostat housing, but i can clearly see another 'unknown' sensor underneath the upper coolant hose, on the housing. Im leaning towards replacing this sensor, or my intake temperature sensor. The gauge fluctuates even if the car is sitting at idle, and trips the engine light sending out the "below thermost stat temperature" code.

any tips? Should i try a new rad cap? Any info on that lower sensor?
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Old 01-09-2011   #7
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Re: Temperature stays cold

lower sensor is just to turn the fan on when the coolant gets too hot. intake air temp has nothing to do with engine temp. so you are getting an engine light when this happens? almost sounds like some sort of problem with the internal processor in the gauges. maybe its not reading the ecu signals from the ect sensor properly. do you have a different gauge cluster you can try? do you have any other strange problems with the car?
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Old 01-09-2011   #8
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Re: Temperature stays cold

i was thinking cluster as well, but surely the computer wouldn't use the cluster for critical decisions like when to allow the transmission to go into lock up. I know the engine (or transmission) used to only fall into final gear once it has been driven for several miles to warm up (AFAIK that's normal behaviour on most cars) When the temp gauge falls to cold the transmission falls out of overdrive sparaticly, again i'm assuming because it thinks its cold. I mean even if i pulled the cluster out the car should still drive and operate normally, no? i know on most cars theres a separate sensor for the gauge cluster and the internal computer. Ive read something about sending units for the temp sensor as well? i think that may be older civics though.

i gotta get to the bottom of this.
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Old 01-09-2011   #9
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Re: Temperature stays cold

you would be surprised lol. there is a computer in the gauges and it also receives signals from the pcm and outputs thru the transmission range switch (which can control transmission gear shifts). the ecu and cluster "drive circuit" are very neatly tied together and it controls alot of different functions.
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Old 01-09-2011   #10
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Re: Temperature stays cold

1. bad cluster
2. bad ECM/PCM
3. short in the wire between the IAT and ECM/PCM connector A10
4. short in the wire between the ECT sensor and ECM/PCM connector B8

Looking at the service manual for P0118, I'm betting on an abraided/broken wire or bad connection somewhere as this would make the cluster read wonky too.

All the ECM/PCM connectors are different sizes so there is no way to get confused. Just count the number of pins and there should be no problem. Here's a pic of the pins and wire colors you're looking for:
Click the image to open in full size.

Service manual says you need a scan tool to properly diagnose. If you have access to a scan tool tell me and I'll post the diagnosis tree. Otherwise I can't narrow it down anymore.
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Old 01-09-2011   #11
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Re: Temperature stays cold

so, what happens if i jump a lead between the two pins? Will that diagnose a bad connection or break in the wire ( by temporarily fixing the issue) or will it cause the car to default to IAT reading for the ECT, or vice/versa?

sorry its not my car so where are these connectors? under the dash or under the hood. PCM is the main computer in most cars, im guessing under the dash?

thanks again!
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Old 01-09-2011   #12
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Re: Temperature stays cold

oh, i should mention that i *may* have tripped the p1811 by messing with the connector while the key was on, as I reset the computer with my basic OBD2 scanner and it has not returned for about 4 days now, but i still get the code "p0128 coolant below thermostat regulating temperature" Fans are not running, happens in town or on the highway, more frequently now in the winter than in the summer...but it happened in the summer as well.
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Old 01-09-2011   #13
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Re: Temperature stays cold

TSB #1003
NHTSA ID #10004966
2002 Honda Civic

Summary: UNDER CERTAIN DRIVING CONDITIONS AFTER A COLD START, THE ECM/PCM CAN MISCALCULATE HOW FAST THE ENGINE COOLANT TEMPERATURE RISES AND SET DIAGNOSTIC TROUBLE CODE (DTC) P0128 OR P1486 (COOLING SYSTEM MALFUNCTION).

hmm..sounds familiar....anybody know more about this?


EDIT:

PCM Software Can Cause
DTC P0128 or P1486
If you’re troubleshooting a ’01 Accord or
’01–02 Civic with DTC P0128 (cooling system
malfunction) or DTC P1486 (cooling system
malfunction) set, here’s something to keep in
mind: The problem may not be the coolant
thermostat, but a deficiency in the ECM/PCM
software. Under certain driving conditions after
a cold start, the ECM/PCM can miscalculate how
fast the engine coolant temperature rises and set
DTC P0128 or P1486.
If the heater and temperature gauge work OK,
don’t replace the thermostat for this DTC; it
doesn’t give a lasting repair. Instead, do this: For
’01 Accords, replace the ECM/PCM (see S/B
01-064, MIL Comes On With DTC P0128); for
’01–02 Civics, update the ECM/PCM software (see
S/B 03-002, 2001–02 Civic: MIL Comes On With
DTC P0128).


it says if the temp gauge is working fine, which mine is NOT. Skip this?

Last edited by nick3501; 01-09-2011 at 05:23 PM.
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Old 01-09-2011   #14
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Re: Temperature stays cold

Yes, skip it. It doesn't concern you if the gauge is working fine.

Connectors are plugged in to the ECM/PCM which is under the passenger dash behind the glovebox. IAT is plugged into the driver's side of the air cleaner box. ECT is the one you changed. Coolant fan switch is the "bottom" one plugged into the thermostat housing.

If your scan tool is two-way then you can probably read live data feed too so, here:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
Sorry about the fuzzy pics, phone cam. Good luck, keep us posted. Pics are nice too. lol
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