Cordial Greetings,

On my '02 S-Type (3.0 V6), I've been experiencing a sporadic Failsafe Mode. The only code ever set is P1584, which points to a fault in the throttle body controller. Usually the Failsafe mode would clear up within a minute or so and the car would run fine, but today it lasted about 10 minutes. As a precaution, I had picked up a spare used throttle body a few months ago. After today's episode, it was time so I swapped them out.

The replacement was a piece of cake. While working under the hood, I noticed a vacuum hose almost severed in half. This was on the 10mm line directly downstream of the throttle body, running aft to the elbow at the aft end of the block. The broken hose connected the plastic line to the nipple on the intake manifold. It was just a short piece of rubber hose about 2" long. I believe this is the line feeding the brake booster. Curiously, I had noticed the brakes seemed to require a bit more effort lately, but nothing too outrageous. For the most part, I think the nearly severed hose was forced shut so the leak wasn't too bad most of the time. The only other driveability issue I've ever had is the car runs just a teensy bit rough for the first 15 seconds or so after a cold start.

So what I'm wondering if there's any way this vacuum leak could have been related to the code P1584. Perhaps the throttle body controller was unknowingly trying to compensate for the vacuum leak, and this somehow set a code? What do the gurus think? I realize it's a long shot, but having two faults at once makes me wonder if they are related. The car hasn't cooled enough to see if the cold start issue is fixed, BTW.

For anybody contemplating a throttle body replacement, the procedure was easy as can be. Disconnect the battery, of course. Remove the flexible duct between the air cleaner and the throttle body. With the engine cool, remove the cap on the coolant reservoir to make sure there's no pressure in the cooling system. The two small coolant lines under the throttle body are a bit tough to access, so I disconnected those two hoses at the far ends, where access is good. Undo the two electrical connectors and four bolts on the throttle body, and catch it as it jumps out. Replace the gasket and you're ready to install the new throttle body. The unit is plug and play, with no programming or calibration required.

The only gotcha is with the alarm after reconnecting the battery. Press the unlock button on your key fob, even if the doors are already unlocked. I didn't realize this and managed to set off the alarm when I opened the unlocked door...

 

Nope, vacuum leak will not set a throttle code. That code is for the throttle position. Those hoses crack all the time BTW.

 

Oops, an update almost three years after the fact: After replacing the throttle body, the P1584 or Failsafe Mode has not returned.

At the time of my original post, I was not aware of the calibration procedure later referenced here:

https://www.jaguarforums.com/forum/s...ection-176446/