Scan code
Joined: Dec 2011
Posts: 120,439
Likes: 17,001
From: Durham, UK
I've moved your question from Jaguar Forums Feedback & Suggestion Centre to X150 forum. The Feedback & Suggestion Centre is for posts about the forum and not for technical questions. This is the place to post technical questions about your model.
You should be serious about anything which suggests an issue with the cooling on a Jaguar V8.
During any prolonged period of storage, the battery should be maintained on a tender. You should ensure the battery is in good condition and fully charged to eliminate the possibility this is a spurious error code resulting from low voltage. Also consider is there the possibility of small animals nibbling at the wiring during storage? In some locations this is a real risk and considerable dame has been reported in several of our model forums.
If these precautions have been observed and it is a genuine code, here are the possible causes of P0128 - coolant temperature below thermostat regulating temperature:
(click on the image to enlarge it)
The first action should be to check the coolant is up to level and top up if necessary. After this I would clear the code, run the vehicle suffciently for warm up and see if the code resets. If it doesn't, you can stop worrying. If it does, then the next likely culprit is a sticking thermostat.
If both coolant level and thermostat operation are correct, then it is on to cooling fan circuit. A fault here should set additional error codes.
Graham
You should be serious about anything which suggests an issue with the cooling on a Jaguar V8.
During any prolonged period of storage, the battery should be maintained on a tender. You should ensure the battery is in good condition and fully charged to eliminate the possibility this is a spurious error code resulting from low voltage. Also consider is there the possibility of small animals nibbling at the wiring during storage? In some locations this is a real risk and considerable dame has been reported in several of our model forums.
If these precautions have been observed and it is a genuine code, here are the possible causes of P0128 - coolant temperature below thermostat regulating temperature:
(click on the image to enlarge it)
The first action should be to check the coolant is up to level and top up if necessary. After this I would clear the code, run the vehicle suffciently for warm up and see if the code resets. If it doesn't, you can stop worrying. If it does, then the next likely culprit is a sticking thermostat.
If both coolant level and thermostat operation are correct, then it is on to cooling fan circuit. A fault here should set additional error codes.
Graham
Last edited by GGG; Apr 23, 2018 at 08:20 AM. Reason: Typo
Veteran Member
Joined: Jan 2015
Posts: 5,281
Likes: 2,555
From: Naperville, Illinois USA
Follow Graham’s advise.
Most likely you’ll find that the thermostat is stuck open. Due to a poor design, the gasket stretches and blocks the thermostat from fully closing, this prevents the engine from warming up as quickly as the computer thinks it should, resulting in the code being thrown.
Not as serious as a stuck closed thermostat and having the engine overheat, but you’ll still want to fix it. It’s the first repair I did on my 07 XK. You should replace both the thermostat and the plastic housing at the same time. If you’re doing it yourself, there is a thread here that has a nice step by step, which you can use along with the steps in the service manual. You can find the parts (thermostat plus housing) online as a complete package, and IIRC it was around $130.
Most likely you’ll find that the thermostat is stuck open. Due to a poor design, the gasket stretches and blocks the thermostat from fully closing, this prevents the engine from warming up as quickly as the computer thinks it should, resulting in the code being thrown.
Not as serious as a stuck closed thermostat and having the engine overheat, but you’ll still want to fix it. It’s the first repair I did on my 07 XK. You should replace both the thermostat and the plastic housing at the same time. If you’re doing it yourself, there is a thread here that has a nice step by step, which you can use along with the steps in the service manual. You can find the parts (thermostat plus housing) online as a complete package, and IIRC it was around $130.
Senior Member
Joined: Jun 2017
Posts: 744
Likes: 235
From: Winston-Salem, NC USA
I second the comments above. I had the same code due to my thermostat being stuck open. kj07xk is absolutely right that if you replace the thermostat you need to go ahead and replace the plastic housing at the same time. Good luck!
Peter
Peter
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