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Richard, Jeep Technician
Category: Jeep
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Experience:  Over 24 years of experience as lead tech and shop foreman.
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U1120 Diagnostic Trouble Code. What does it mean Just purchased

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U1120 Diagnostic Trouble Code. What does it mean? Just purchased used 06 Commander with all the whistles, "perform service" light shows a few times then goes away. Engine symbol steady 1st few days of driving. Inspection revealed this code & P0456 which I saw was "evap system leak - minor". Please help

Hello and welcome to Just Answer!

U1120 means the PCM doesn't receive a vehicle speed signal from the Anti-lock brake Module over the CAN C bus. This can be caused by the ABS module, PCM or something wrong with the can bus. This might be just an erroneous code and may never set again.


PO456 is a small evap leak, which is usually caused by a loose/not sealing gas cap. To test this out properly an evap smoke machine will be needed to locate the leak. If you want to try a new gas cap and clear the codes, that might be all that is needed. The PCM checks for a small leak on an overnight soak or when it sits for a certain amount of time. I have seen other causes for the P0456, but the gas cap is usually the cause. Without having it tested there will be no way to know for sure where the leak is.


I would suggest maybe trying a gas cap and clear the codes to see if that takes care of the problem.


I hope this helps, anymore questions feel free!

thank you,


Customer: replied 6 years ago.

Sorry for delay! I'm new at this, but I will accept your answer after your opinion on what happens next. The re-inspection told me he couldn't get the light to go out. He told me to take it back to the dealer & have them do what they have to. The dealer offered to have the car taken to their mechanics to fix the problem. I'm assuming the CAN C bus you mentioned could be the worst case scenerio? He (the dealer) mentioned transmission when I read the code error to him. Should I be concerned? Oh, by the way, it was the "new" gas cap w/ lock I put on car causing other code. THANKS


PS - car has 98k & 30 day warranty ends 6/25/10

Is the U1120 the only code that is setting now?


Is the ABS brake warning lamp on?


U1120 will not be caused by the transmission!


Customer: replied 6 years ago.
The code is still there. I gave it to the dealer who said he would order some computer part or something to clear everything. When I got the car back, the light was out but still said "perform service" & "check gas cap", then drove a couple of days and engine light came back on. No ABS brake lamp on, but breaks are squeaking a little, should I make them change that too?

It sounds like they never fixed the emission leak that was causing P0456. I would take it back to them and fix it right this time. Emission leaks aren't that hard to find and repair. The dealer will have a special machine to check for a small leak and chances are, they never even hooked it up and just replaced what they thought was wrong.


There is probably nothing wrong with your brakes, but it wouldn't hurt for them to check them and at least tell you how much brake pad is remaining in the front and rear in 32s of an inch. Brake squeak and squeal is a common concern and usually there is nothing wrong.


Let me know how you make out!

Thank you,


Richard, Jeep Technician
Category: Jeep
Satisfied Customers: 4139
Experience: Over 24 years of experience as lead tech and shop foreman.
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