DTC P0128 – How To Install a 350Z Thermostat

The OBDII DTC of P0128 is a bit unique of a check engine trouble code. While most of the time, temperature related OBDII diagnostic codes relate to heat, or overheating. Instead the P0128 trouble code is for the engine being too cold, or your thermostat failing to warm up the engine to operating temperatures.

The thermostat in your Nissan 350Z is responsible for keeping the coolant inside the engine, and preventing the coolant from flowing into your cooling system. When the temperature of the thermostat has been reached, the thermal wax coupler atop your VQ35 thermostat opens, allowing coolant to flow into your radiator.

When your Nissan thermostat is stuck open or has a tear or leak in it, your 3.5 liter engine can fail to warm to 180ºF. If you’ve checked your check engine trouble code in your 350Z, and your scan tool returns a P0128, you will need a new thermostat. Today I’ll be showing you how to install a 350Z thermostat in a 2005 Nissan 350Z with a VIN ID of X.

This vehicle is equipped with a 3.5 liter VQ35DE engine, which is also found in the Infiniti G35. The part number for this Nissan thermostat is 21200-31U13. If your local Nissan dealer is out of these thermostats, you can opt for a Infiniti version under part number 21200-4W01B or 21200-31U13A.

Before you begin our how to install a 350Z thermostat DIY Guide, you will want to check a few things. Primarily that your engine is cold, and your coolant isn’t hot to the touch. Because you’ll be removing the VQ thermostat and thermostat housing, the chance of hot coolant injuring you is high.

Open your hood and drain your Nissan radiator completely. For the trouble code of P0128, we recommend that you always use a genuine Nissan thermostat as a replacement. Normally we would suggest a lower rated thermostat for your VQ35, but in this case, you’re best served by sticking to OEM parts.

Step 1 – Remove your 350Z Intake pipe and MAF

Disconnect the negative terminal to your battery by opening your battery tray in your 350Z engine bay. Once you have this disconnected, unplug your Nissan mass air flow sensor.

Remove the 10mm nuts that hold the top of your engine cover in place. Remove the plastic VQ35 engine cover and set it to the side.

Remove all of the plastic scoops and ducts that connect your air box to the front of your 350Z.

Now remove the 10mm bolt that runs through the top of your intake tract and into the upper intake manifold plenum. Loosen the worm clamps that hold the accordion coupler in place, and remove the MAF, air box assembly as well as the second half of your intake pipe.

With all those things moved out of the way, you are ready to learn how to install a 350Z thermostat. At this point your radiator should be drained, so go ahead and remove the 10mm bolts that hold the thermostat housing to the engine.

Pull off the thermostat housing, which should also include the thermostat itself.

Hopefully you aren’t burned at this stage, because there will be a little excess coolant left over in this orifice. Now completely clean and dry the area, as you’ll be reinstalling the new 350Z thermostat and sealing with RTV gasket maker.

Install the new 350Z thermostat by using a new gasket and applying a thin layer of hi-temp gasket maker. Gently reinstall the thermostat and tighten the bolts to specification, allow the thermostat housing gasket maker to dry before pouring in new coolant.

Q : I installed a new thermostat in my 350Z, but my P0128 is still there.

A : While it’s not typical, the engine coolant temperature sender in your VQ35DE could be bad and / or sending wrong information to your Nissan ECU. This two prong engine coolant temp sensor (ECT) can be tested to see if it’s still in proper working condition.

The part number for this Nissan ECT is 22530-44B20, 22630-0M200 or 22630-43Y00. You can also find this ECT sensor under the following part numbers :

• 22630-44B10
• 22630-44B20
• 22630-71L00
• 22630-7Y000
• 22630-ED000

Can’t find it under a Nissan or Infiniti part number? Not to fret, there’s a few Ford interchange part numbers as well ; F6XA-10884-AA and F6XZ-10884-AA. To test your Nissan 350Z engine coolant temperature sensor, remove it and measure the internal resistance using your multimeter.

Not sure how to use a multimeter? Look at our How To DIY guide here for a few pointers on how to test resistance using a multimeter. With the engine coolant temperature sensor at room temperature, you are looking for a resistance range between 2.1 and 2.9 kΩ.

With the Nissan ECT at 122 degrees, you are looking for a resistance range between 0.68 and 1.00. If your ECT returns an internal resistance that’s outside this range, it’s time to replace your ECT and clear your OBDII codes.

You now know how to install a 350Z thermostat and how to resolve the P0128 OBDII check engine code.