P0128 - Just changed the freakin' T-stat!!!

[osufans]

Love it...after the whole debacle of getting the wrong radiator, being without my Jeep for a whole friggin' week, and finally getting it all back together.....today I get a CEL, P0128.

I know what it is...and I can only assume it means the BRAND NEW t-stat I put in is bad. I'm just checking to make sure.

Is it even possible to put the thing in backwards? I distinctly remember which way it came out, and put it back in the same way...heck, there's even a notch in the t-stat housing for the t-stat bypass valve, so I know it's in correctly.

Just can't believe I'm gonna have drain all the fluid so I can pull the new t-stat and put another one in.....uggh!

 

[Ry' N Jen]

I too have had this happen in the past. (Not on our KJ, but on another make model.)
Before you install the replacement, toss it into a pot of boiling water and make sure
the "new" one opens and closes.

 

[osufans]

I too have had this happen in the past. (Not on our KJ, but on another make model.)
Before you install the replacement, toss it into a pot of boiling water and make sure
the "new" one opens and closes.

I will be filing this one away....never had to do this before...but especially on vehicles where you have to drain the entire system just to replace the t-stat...I will definitely be doing this in the future.

 

[tjkj2002]

Love it...after the whole debacle of getting the wrong radiator, being without my Jeep for a whole friggin' week, and finally getting it all back together.....today I get a CEL, P0128.

I know what it is...and I can only assume it means the BRAND NEW t-stat I put in is bad. I'm just checking to make sure.

Is it even possible to put the thing in backwards? I distinctly remember which way it came out, and put it back in the same way...heck, there's even a notch in the t-stat housing for the t-stat bypass valve, so I know it's in correctly.

Just can't believe I'm gonna have drain all the fluid so I can pull the new t-stat and put another one in.....uggh!

Code P0128 does not always mean a bad t-stat,but yes in most cases a stuck open t-stat is the main problem and just because it's "new" does not mean it is going to work right.

 

[osufans]

I know the gauge on the dash isn't analog, but it does come up to temp and sits right in the middle like before.

I don't recall any other sensors that would've been disconnected during the rad. install. I'm pretty much figuring it's the t-stat, unless someone else has any suggestions.

Thanks!

 

[osufans]

OK, so have another question about this issue.

The day I was planning to change out the t-stat...the CEL goes off. I think cool, maybe there was air somewhere...problem solved. Now, a few days later, it came back on again.

There has been a difference in the ambient air temp between the time it first came on (~20*F), the time it went off (~40*F) and the time it came back on (back around 20*F). So I'm guessing it may still be taking too long to come up to temp. However, I don't notice any issue with it warming up, heater still works good, and the engine temp sits right at midline all day long.

So my question is, since it's such a PITA to get to that housing, plus having to drain all the fluid, what does it really matter if I don't swap out the t-stat?

 

[tjkj2002]

OK, so have another question about this issue.

The day I was planning to change out the t-stat...the CEL goes off. I think cool, maybe there was air somewhere...problem solved. Now, a few days later, it came back on again.

There has been a difference in the ambient air temp between the time it first came on (~20*F), the time it went off (~40*F) and the time it came back on (back around 20*F). So I'm guessing it may still be taking too long to come up to temp. However, I don't notice any issue with it warming up, heater still works good, and the engine temp sits right at midline all day long.

So my question is, since it's such a PITA to get to that housing, plus having to drain all the fluid, what does it really matter if I don't swap out the t-stat?

It just effects your mpg's and how long your cats will live.

 

[osufans]

It just effects your mpg's and how long your cats will live.

So I was wondering about that, because I know a cold engine will run richer...but I guess what I'm getting at, once the engine comes up to temp, does the t-stat ever close? Like does it actively regulate temperature?

My point is, the engine comes up to temp just fine. I don't notice any ill effects of having this CEL. If the t-stat does indeed open and close regularly to maintain operating temperature within a range (say <195*), then I could understand the decrease in MPG's and toasting the cats....however it would seem to me that once the temp hits 195*, it stays above that mark. Dunno, just wondering out loud here.

Thanks!

 

[tjkj2002]

So I was wondering about that, because I know a cold engine will run richer...but I guess what I'm getting at, once the engine comes up to temp, does the t-stat ever close? Like does it actively regulate temperature?

My point is, the engine comes up to temp just fine. I don't notice any ill effects of having this CEL. If the t-stat does indeed open and close regularly to maintain operating temperature within a range (say <195*), then I could understand the decrease in MPG's and toasting the cats....however it would seem to me that once the temp hits 195*, it stays above that mark. Dunno, just wondering out loud here.

Thanks!

Your temp guage is a dummy gauge,when at the center mark your engine temp will be anywhere between 178 degrees through 239 degrees and the gauge will read the same anywhere in that temp range.

 

[osufans]

Your temp guage is a dummy gauge,when at the center mark your engine temp will be anywhere between 178 degrees through 239 degrees and the gauge will read the same anywhere in that temp range.

I'm not just talking about the temp gauge...the air coming out of the heater gets hot just fine as well. I see no difference in the Jeep before and after, including MPG's (which actually have increased +1MPG since I've swapped the radiator....could've been the weather.)

I guess I was back to wondering if there was something else that could be the culprit for the CEL.

To note...the coolant level has not changed any. Everything looks dry and tight.

 

[diyman]

Ehmmm.... We all are assuming the ECT sensor is OK, isn't it....!?

Just thinking out loud, but, what if the sensor is wrong, that is it's not reporting the right temperature?

A scan-tool in live-data mode might help. No need to be an expensive one (better if borrowed ) because it
only reports what the PCM 'thinks' ( by means of the ECT sensor) is the coolant temp.

It's just a remote possibility, but :shrug:

 

[osufans]

Ehmmm.... We all are assuming the ECT sensor is OK, isn't it....!?

Just thinking out loud, but, what if the sensor is wrong, that is it's not reporting the right temperature?

A scan-tool in live-data mode might help. No need to be an expensive one (better if borrowed ) because it
only reports what the PCM 'thinks' ( by means of the ECT sensor) is the coolant temp.

It's just a remote possibility, but :shrug:

Where is that sensor? I'm just wondering if I unplugged something and forgot to reconnect it in my haste to get this thing back together (after being apart for a week.)

I looked over it again this afternoon and didn't see any loose connectors floating around anywhere....who knows at this point.

 

[diyman]

I don't mean it's disconnected ( there should be another DTC in this case, I suppose...), I just thought the sensor 'might'
have aged thus giving bad readings...

IIRC changing it (in case of...) also means spilling some coolant. But hey, I was just throwin' in another clue..
And no, You probably didn't even get close to it while changing the t-stat, don't worry about that.

 

[tjkj2002]

Where is that sensor? I'm just wondering if I unplugged something and forgot to reconnect it in my haste to get this thing back together (after being apart for a week.)

I looked over it again this afternoon and didn't see any loose connectors floating around anywhere....who knows at this point.

Look behind the alternator,the ECT sensor is the tiny sensor in that spot.

Oh and your heater temp has really nothing to do with a good,bad,or lazy t-stat.