02 Cxl P0128 Service Engine Soon Code

2002 CXL 98K 3.4L
Around Thanksgiving my SES came on. Autozone read the code as:
P0128 Generic Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature). The autozone guy said my Coolant Temp sensor was going bad and recommended replacing it (I replaced sensor). At that time my Temp gauge was showing a lower than normal reading, meaning it was running cooler than I thought it should. Of course it is Michigan and November so it's hard to be sure. Fast forward to Christmas SES comes on again. Same Code. The other Autozone guy says the first technician mis-diagnosed the first code and says that either my coolant is to low or my theromstat is opening to soon. I was low on coolant (below Minimum) I topped it off and are playing a wait and see game. I asked the technician about clearing the code and he said every 25 key turns the vehicle performs a self diagnostic and at that point the code should go away if the low coolant was the problem. Anybody want to offer some opinions or guess's on what they think of this problem?

Sidenote: I double checked for a coolant leak and have not found one and still have plenty of heat.

First of all AZ is not the place for repair advice, period. Second this problem coupled with a lack of sufficent heat from the heater almost always points to a bad thermostat.

Is the thermostat replacement a DIY job. One forum mentioned about having to remove a crossover exhaust piece. I'm assuming it's buried in the engine comparment and I will have to take it to a mechanic.

On the 3400 engine the thermostat can be replaced DIY but some parts do have to be removed to get to it and you need to use the proper tools. Just removing the crossover pipe heat shield helps for a little extra clearance and many remove the throttle body. I've never had to remove the complete crossover pipe. Midget sized hands may help too. :uhoh:........:lol:

But I agree with Shep in most cases the P0128 DTC is caused by a bad thermostat (opening too soon or stuck open) and AZ is in the business to sell parts not give expert tech advice.

Thank you both for your help. That's why I always comeback to Automotive Forums. I definitely learned a lesson about AZ. Its a typical January here in Michigan (really fricking cold). I will probably just take to a mechanic. I don't have a garage to work in. Hopefully I can get out of there for $100 or close to it.

I hope you can do it yourself, I had to take mine in and it cost me plent. Haynes guide calls for 3hours labour so that is what I was charged.

Remeber the good 'ol day's when a $3 part was easy to change!:banghead: :banghead:

I have a family member who is going to give me a hand doing this. He has the heated garage,tools and knowledge. Besides the theromstat (gasket too?) is there anything else I need to purchase? When I remove the throttle body is there a gasket that needs to be replaced for that also?

I called 3 local mechanics and got a price quote to replace a theromstat. Low ball price was $130-$150. Middle was $225-250. High was $323 out the door. I wonder if the low ball knew there was some work involved to do this job or was he just quoting a normal (normal being without having to tear down half the engine) thermostat replacement.

Is'nt owning a "newer" vehicle a whole lot of fun!

Having opted to help support my local mechanic, I can only assume the obvious items like the thermostat housing gasket and a tube of silicon. I would see if there is a flang gasket or something similar needed when removing the crossover.

Have fun and make sure you have plenty of beer on hand. I would rate this one at least a 12!

In my first post I had mentioned that the "expert" advice I received from the Autozone guy was that every 25 key turns (or cycles) the computer does some self diagnostic and if my coolant being low was the problem the SES light will turn off.

Well...the SES light is now off.

I was always low on coolant. Meaning the reserviour was below the minimum level. It was that way for a while. I was just to lazy to fill it up. After a couple trips to AZ I added coolant and was planning on changing out the thermostat. I was banking on that this was not going to be an easy fix of just adding coolant. Was I wrong? Common sense tells me that something else has to be going on to generate P0128. But for now it's off. I guess I can play the wait and see game and watch to see if it comes on again.