***I am not a mechanic, consult an experienced mechanic for auto repairs.  This is just informational***

My previous repair of the ECT Sensor did not fix my 2003 Nissan Maxima.  After driving around, the check engine light did not go away, so that means I needed to go to the second potential problem and more difficult repair, replacing the thermostat.  The repair would not be difficult, but the genius engineers are not designers.  So you will see in the guide below, that you would be working in limited space, should you need to do this repair, and that is what makes this a more difficult repair.

2003 Nissan Maxima Code P0128

Code P0128 is an error code related to the cooling system.  The possible problems from my print out are either the thermostat has gone faulty and is either stuck closed or open.  This can lead to the engine either over heating if stuck closed, or not warming up enough if stuck in the open position.  Another possible problem is the Engine Coolant Temperature Sensor has gone faulty.  The engine coolant temperature sensor  (or ECTS or CTS) is used to detect the engine coolant temperature.  Information on the ECTS can be found in the FSM (factory service manual) in section EC-207 and EC-219.  It has procedures for checking the part, but since it’s a cheap part to replace and the car is nearing 17 years old, I figured I would replace this first and see if it fixes my check engine light (it did not fix it, but at least it’s one less worry).  The second possible repair involves the removal of the thermostat, which has a stupid design in this car.  The thermostat is integrated in the water outlet and has 3 bolts, one of those bolts is in a very poorly designed location.  Information on thermostat replacement can be found in section LC-18 of the FSM, although not very detailed, and you will also need to drain your coolant and fill it back (information located in section MA-14 through MA-15).  Also, if you need an FSM for a 2003 Nissan Maxima, I downloaded a copy here.

Tools Needed for the Repair

So, obviously you need the new thermostat (about $20 from RockAuto) which is in the rear of the outlet on the part to the left, I have a gasket which is the cardboard (included with thermostat) under the Permatex Water Pump/Thermostat Gasket Maker (about $2 on Amazon, and cheap insurance against a leaky gasket), a flathead screw driver to help in getting a couple of hoses off, a Philips screw driver (not pictured) to drain the coolant, a ratchet, various extensions, you definitely need a swivel for this job, 10mm sockets (regular and deep), pliers for the hose clamps.  I only had a 3/8 ratchet, but would highly recommend a 1/4 ratchet.  That is on my list to add to my tools the next time I go to harbor freight.

Instead of reusing my old coolant, I figured it was a good time to put fresh coolant in the car.  So I purchased 3 gallons at wally world for just under $30.  I can return 1 gallon if I don’t need it.  You will also want something to catch the coolant when you drain it, and also don’t dump the coolant.  Put it in some empty water and milk jugs and take it to a local repair shop so it can be recycled.  Also helpful is to have some cardboard or old newspaper for when you drain the coolant, just keeps things cleaner on the ground.

The Part

This is the new water outlet.  The thermostat is integrated into this part on the rear.  The main thing to notice are the positions of the 3 bolts.  This is about the best view you will have of the bolt locations on this job.

Replacing the Thermostat

First you need to find the thermostat housing location, which is pretty easy.

Looking at the engine bay, the thermostat is located on the passenger side, indicated by the arrow.

From the passenger side, you can kinda see the part from this picture.  It’s pretty dirty.

Above is a zoomed in view of the thermostat housing, highlighted with my laser pointer.  You can see 2 of the 10mm bolts that need to be removed.  The third is located underneath the elbow.

The first thing I did was go under the car and remove the wind deflectors under the car.  You can skip this part, but I wanted the coolant to drain straight into my pan and not take a chance of it draining in the deflectors.  The can be removed with a 10mm socket wrench and there are also some of those push in fasteners.

You can remove just the passenger side, as the drain plug is located there.  If you choose not to remove this panel, you can still located the drain plug and there is an access hole in the panel.

Here is a shot of the drain plug, after I drained the coolant.  Basically loosend with a phillips screw driver, the remove the radiator cap to get the coolant draining into your pan.  Don’t forget your cardboard underneath as well.  Just keep an eye on your drain pan.  As it fills up swap it out for another one, and place the coolant in an empty water jug.  Just kind of chill for a while, while all the coolant drains from the system.

This pic shows the approximate location of the drain plug from above.  Basically the location of the rubber grommet for the radiator on top, if you follow that straight down, you will see the drain plug from below.

Before I removed the thermostat housing, I went and grabbed some rags and a zip lock bag.  I wanted to be careful when removing the part, some leftover coolant may drip out.  This part is located almost directly above the alternator.  So I put the zip lock bag over the alternator to protect it, and had rags ready as well.

First you should remove the hoses. So get the pliers and flathead screwdriver to try and pry off the hose.  It’s been there for 17 years, so it won’t move without a fight.  Second remove the lower radiator hose.  Same thing, pliers to move the clamp away, then use the flathead to help remove the hose.  Tuck both hoses out of the way.  This gives you easy access to the top right bolt. For the bolt to the left, you can either release the tensioner for the belts to get easy access to the bolt.  I didn’t want to mess with my belts, so I used a swivel on my ratchet and a regular 10mm socket.  Once you crack the bolt loose, you can easily remove the bolt with your fingers.  For the third bolt, I used a 10mm deep socket and swivel to remove the bolt.  the bottom bolt is a little tricky, so take your time and curse if you have to.  For removing the bolts, prepare to swap between various extensions and the regular and deep socket.

With the 3 bolts removed, you can take out the part.  Don’t forget to remove the old gasket, and also clean the surface as well before installing the new part with the gasket.  I also used permatex water gasket/thermostat maker to be safe against having any leaks.  This was not a fun part to take out and I don’t want to have to do it again.

Installation is the reverse of the above steps.  It will be tricky to install the part, because you will have a hard time lining up the new part, with the gasket to the holes for the bolts.  You are kinda working blind on this one.  I was able to hand tighten the lowest bolt first, followed by the bolt to the left, and had to feel around for the top right bolt.  If you are able to get a torque wrench in the tight workspace, the FSM says the bolts should be tightened down to 74.8-99 in-lbs.  I used Permatex anti-seize on the bolts as well.

Put the hoses back on after all the bolts have been tightened down.

Everything is back in place and the new shiny part can easily be seen.

After the part is installed, allow time for the gasket maker to dry (Permatex recommends 24 hours).  Then you will have to fill and bleed the radiator to remove any air bubbles from the system.  Nissan did not install a bleeder valve on this car for some reason.

I bought this nifty funnel for $25 on amazon.  It basically has an empty shell of a radiator cap, with a plug that you connect the funnel to.  Pour in the coolant and don’t let if ever get empty or  you have just let air enter the system.  Squeezing the top radiator hose helps to release some air, before you start the car.  Basically you start the car, turn the heater all the way to high and the fans high.  You will see bubbles come up the funnel.  Also getting in the car and revving for 10-20 second increments will help burp the air out.  If you initially only feel cool air coming out of your vents and the heat is on, don’t worry, the air needs to work it’s way out and then you’ll be really feeling the heat.  Just remember to keep the funnel about 1/3 full.  After no more bubbles are coming up, turn off the car and let everything cool down.  Keep an eye on the coolant level in the funnel as it will go down as the coolant cools off.  I also ran the car in the driveway again, letting the car warm up and wait for the radiator fans to kick in a couple times.  When your done, don’t forget to add coolant to your overflow reservoir and place your radiator cap back on and check for any leaks.  Then it’s time to test drive on the road, keep and eye on your temperature gauge and if the car is getting hot, pull over and let the car cool down.  For the next few days, check for any leaks, and watch your coolant level.  It may be necessary to add coolant and burp out some more air.  The check engine light should go away after putting on some miles.  You can also go to the local parts store and they can clear the light for you.  The light would come back if not fixed.

The total cost of this repair was $20 for the thermostat, 2 gallons of coolant at $9 each, Permatex for $2, $25 for the funnel, for a total of $65.  I’ve checked online and the parts and labor are about $300, the shop would also hit you up for about $100 to fill and bleed your coolant, so you could easily be looking around $400 for this repair.

Something I wish I did was order new radiator hoses.  For about $20 bucks and not much more time, I could have replaced the old hoses and had one less potential problem down the road.  Also, it would save me time from having to bleeding the coolant again.

Update on 1/11/2020: My check engine light went away after about 150 miles of driving.  I noticed while driving during the week how much quicker the car was warming up by looking at my temperature gauge.  Also my MPG went back up to normal (30MPG).  This is because the car was previously not warming up properly due to the thermostat being stuck open.  This caused the car to burn more fuel, to try to get to normal operating temps.  The drop in MPG was about 2 MPG.

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