[modern] Check Engine - CODE 17

As I started my 90 XJ40 for the trip home from downtown DC,
Thursday, I was greeted with a “CHECK ENGINE” alert on the
dash. Naturally I thought it must be a mistake and immediately
shut the engine off and restarted, only to have it return. OK, time
to drive home. Later, consulting the owner’s manual and Haynes,
I discovered the problem was the throttle position potentiometer
(tp-pot). On Saturday, I looked things over, comparing Haynes to
the view under my hood (bonnet). I used contact cleaner
(not WD-40) on the contacts of the plug-recepticle for the pot.
No change. Today (Sunday) I went for broke and meticulously
followed Haynes page 6-6 & 6-7. As expected, my plug conforms
to the 1990-1994 configuration and I used a pin to “backprobe”
measure voltages as described. They were close but the low
end was a bit too low. I removed the entire throttle body, conducted
a routine cleaning, and removed the tp-pot for inspection. It looked
OK & had smooth action, so I measured the resistance. It seemed
smooth with reasonable resistance change with movement. So, I
reinstalled it on the throttle body and adjusted it as described in
paragraph 34. Bolted back together I had .504vdc - throttle closed
and 4.85vdc throttle open. I started the engine to only to get the
suspected “CHECK ENGINE” alert. I drove if for about 5 miles at
speeds up to 50 mph or so, since Haynes page 6-3 tells us the
code will reset at 19 mph. Mine didn’t reset but the engine still
ran great. So, I followed the procedure for 92-94 models and
disconnected the battery. I left it for about 15 minutes while doing
other things. After reconnecting, this start-up produced a clear dash

QUESTION: Is Haynes wrong? Did I misread? Do I have an
unusual car? Is there a better/other way to reset?

Responses/advice appreciated.

Harry Elam
90 XJ40 109K miles – 66 Sunbeam Tiger 178K miles