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Dan, ASE Certified Technician
Category: Car
Satisfied Customers: 1686
Experience:  Auto Technician
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My gmc serra truck is not changing from 4 cylinder to 8

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My gmc serra truck is not changing from 4 cylinder to 8 cylinder smoothly. I changed out both motor mounts. But still having problems.

Hello my name is***** am an ASE Master Technician and I am glad to help you out with your question.

There is a tsb for this if I recall, send me the vin and I can look this up for you in the service manual.

Customer: replied 10 months ago.

Do you have any codes? have you had it scanned?

Customer: replied 10 months ago.
I had it scan at GMC dealer. I was told the saner said motor mounts had to be replace. I already had replace one. So I replaced the other one. Check the existing moulin and it look good. I had replace the other one about two months ago. I'am still having problems going between 4 and 8 cjlinder. And the dealer didn't tell what codes came up.
Customer: replied 10 months ago.
When can I expect an answer!

I am sorry for the delay, I work at the dealership full time as a tech and try to get on here as soon as I can. Take this to any auto parts store and get a free scan. Then write down any codes. Let me know what you come up with. Thanks.

Hello again Dan here,

I am contacting you because the post has not been closed or rated.

This question is coming up on 7 days which means it will time out and close.

You have already paid to get this question answered, but you have not rated.

Let me help some more. Is there anything else I can help you with?

If I did not give you 5 star service, please let me know what I missed.


Customer: replied 10 months ago.
I had another computer scan performed by someone else. Data is showing as PO121.
Throttle/Petal position sensor
ECM code

The most common cause for this code is a faulty throttle body.

Customer: replied 10 months ago.
But does this cause the problem on my truck to change from 4 to 8 cylinder and run so rough.

It could cause a rough running problem, yes I would start with diagnosing this code first. You could also try cleaning the throttle body and clear the codes.

Customer: replied 10 months ago.
Will get back with you soon.

Ok thanks.

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