Was driving on highway and decided to engage the cruise control. I set it at 65, felt the car take hold of it momentarily, then noticed cars passing me. Glanced down and saw speed was decelerating. At second glance, I noticed the car put itself in safe mode. I disengaged the cruise control and got to where I was going with no further problems or noticeable issues. When I restarted the car, it was back to normal.


Haven't tried to use the cruise control since, and don't intend to until I get this addressed. Is there a reason other than the lack of cruise control I should address it sooner than later?

 

There should be a DTC in a module. Brake switches, throttles, or some other module fault on the CAN bus can cause that type of symptom.

More diagnosis.

bob gauff

 

Since the cruise control is vacuum based, you may have small leak on the hose which goes to the throttle body. Check that first.

 

Try removing the vacuum hose from the dome on top of the throttle and you should get an engine fault and P1235 code but not safe mode, at least not on my XK8.

 

+1 for a start at checking.
In my case the garage forgot to install back the vacuum line on the throttle body. I got SAFE mode as soon as the cruise control had to accelerate the car (like hill).