Codes P0128 and P0126, coolant thermostat?

CHRISTOPHER NELSON2

MEMBER
2012 JAGUAR XJ
5.0L
V8
2WD
AUTOMATIC
85,875 MILES

My engine light came on two months ago. I went to have a diagnostic code checked and it pointed to the Coolant Thermostat. So, I ordered one and had my mechanic replace the failed one. He showed me the thermostat that came out and the internal flap was busted. About 40 miles later the engine light came back on, and I took the car back to him. He asked me if I purchased the Thermostat from Jaguar and I said no, I purchased it from RockAuto which was a direct OEM replacement. Could this be true, as I believe it may be the coolant temperature sensors. The engine temperature need slowly show the car at normal and when it does reach operating temperature, sometimes it drops slightly below normal temperature and then returns to normal. Which do you think it is? Should I purchase directly from Jaguar or replace the Coolant Temperature Sensor/Sensors?

Wednesday, June 11th, 2025 AT 10:16 AM

1 Reply

JACOBANDNICKOLAS

MECHANIC
110,144 POSTS

Hi,

Both codes are related to the thermostat. I attached two pics below so you could see the definition for each code and the possible causes.

Basically, the codes indicate the engine isn't reaching operating temperature fast enough. As a result, the vehicle is producing more emissions than it should.

In this case, it could be either the thermostat or an electrical issue with the coolant temperature sensor. However, if I had to speculate, I would say replace the thermostat. Note that a direct fit part only means it will fit. How it functions is something different. I feel your mechanic is likely on the right track, saying it is a faulty thermostat.

Let us know if this helps or if you have other questions.

Take care,

Joe

See pics below.

Images (Click to make bigger)