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Discussion Starter #1
Recently my service engine light came on and the P0128 code was read. Based on my research I understand this normally means that the thermostat has failed, and usually in the open position. However, in my situation specifically, the light came on right in the middle of performing a coolant flush, so I'm wondering if that's what is causing the code and not actually a broken thermostat.

I had just flushed the radiator and refilled with distilled water and that Prestone Radiator Flush+Cleaner fluid, and had been driving around a couple days to thoroughly clean the system in preparation for a flush and fill with new coolant. Then the light came on. I assumed it must have come on due to running on distilled water and the cleaning fluid, so I cleared the code and performed my coolant flush (kept flushing with distilled water until it flushed pretty clear and then added coolant to achieve a near 50/50 mix). After driving for a day or two after the coolant flush, the light came on again, same code.

At this point, I'm wondering if the thermostat is indeed broken and that somehow my coolant flush triggered it breaking or something, not sure. I also have been monitoring my coolant levels the last couple days and topping off each morning while cold as the air bubbles are let out, so I may try and clear the light one more time once I know the air is out of the system and see if it comes back again, in case that was the reason.

I also used my OBD2 scanner to read the Engine Coolant Temperature (ECT) sensor while the car was cold, and it read exactly the same as the ambient temperature, so I do not think the sensor has gone bad (which could have been another reason for the P0128 code), so I can rule that out.

Additionally, I turned the engine on and drove around for 20-30 minutes and the ECT sensor temp got up to 79 degrees Celsius, mostly hovering around 75 (167 degrees Fahrenheit), which seems concerning as I believe that is a little low for normal operating temps (190-200 degrees Fahrenheit I believe). The ambient temperature was around 42 degrees Fahrenheit today while I was driving around.

I got my X at the end of September and I'm thinking that maybe the thermostat has BEEN broken in the open position but that since it was not really getting too cold until recently, the coolant was able to reach higher operating temps with the thermostat open anyways, so it would not have triggered the P0128 code until now when it is colder.

I'm hoping it's not the thermostat and that it has something to do with my recent coolant flush and air in the system and that it does not turn back on after I clear it this time. I know a thermostat is cheap but I just really don't feel like replacing it right now, especially with all the horror stories I've read of stuck on gaskets and how annoying the thermostat is to get to. I also just flushed my coolant and it would be a waste to have to flush it and replace it with more new coolant while replacing the thermostat, but coolant is relatively cheap anyways I guess. I just don't like being wasteful.

If I do end up replacing the thermostat, do you guys recommend I change the sensor too? Maybe the belt while I'm down there? Rad hoses? I've done a decent amount of work on my X so far and I'm hoping this will be the last thing for a while.
 

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I had an 03 xterra that I replaced the rad and both hoses in and I didn't replace the thermostat. Bad move. ended up replacing that later on down the road. I also had on 03 mustang gt that had a bad water pump. I learned my lesson the last time and just replaced the thermos while I had the coolant system taken apart.

In my opinion replacing the thermos whether its bad or not during a coolant system job is a no brainer. They're like, what, a few bucks? and it only requires a few extra steps... Also, running at a cooler temp probably wont hurt the truck so don't sweat it too much.

Might as well do some preventative maintenance while you're down there huh? ;)
 

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Discussion Starter #3
Yea I’m definitely glad my temps are running low and not high, so the engine will be fine but I’ll lose some mpgs/performance, as it will take more effort for the engine to reach operating temps if the thermostat is stuck open.

I was just doing a coolant flush so I didn’t have anything undone/opened, but yea a new OEM thermostat is only 20-30 bucks. It’s just a potential bitch to replace in the Xterras.
 

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Yea that is true... I haven't done any coolant work on my truck so I cant really have an opinion on the difficulty. I've only had my 14' a couple months so I haven't had the chance to do anything haha...

The thermos should just be right at the end of the upper rad hose where it meets the block. should be some sort of flange and a gasket... Just unscrew bolts clean gasket surface and add new gasket maker. Some thermos from auto stores come with a little gasket you can just pinch between the block and the flange. Try to find one like that so you don't have to mess with rvt. That stuffs a PITA.
 

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Word of advice, if you do end up replacing your thermostat... be sure you use OEM. Aftermarket thermostats are hit-or-miss and mostly miss. Save yourself from having to do the job twice and go with OEM.
 

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Discussion Starter #6
Yea definitely going OEM, any advice on gaskets? I wanted to get the Stant gasket that comes with one side sticky, so I could place it easily, but it has some bad reviews. Looking at the Beck/Arnley one now.
 

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When I did my thermostat, I bought the OEM gasket along with the thermostat from the dealer since it was easy and available.
 

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Discussion Starter #8
When I did my thermostat, I bought the OEM gasket along with the thermostat from the dealer since it was easy and available.
Do you have any recommendations for putting on the gasket? Do I need that "RTV" sealant stuff or is the gasket enough?
 

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I just scraped off the old gasket and put the new one in it's place. Didn't mess with RTV. Didn't see the need. No leaks and it's been over a year.
 

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Discussion Starter #10
As an update and FYI for future people reading this thread, I've determined my thermostat is indeed broken and stuck in the open position, allowing coolant to constantly flow freely through the engine and to the radiator without reaching the optimal temperature.

I was able to determine this because I idled the car for 15 mins and monitored the coolant temp (using my OBD2 scanner, you should get one!). It never got near the temp required for the thermostat to open up, so it should theoretically have stayed closed, but then I felt the top of the radiator and it was very warm, meaning coolant was flowing through the radiator from the engine. That’s why it’s not able to warm up to the optimal temp in time and then throws the P0128 code.

Ordered an OEM thermostat and gasket and will be replacing this weekend!
 
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