Hmm, I absolutely love the fact that you have 490,000 miles. Kudos.
Really don't have any clue why you're getting this, except, I'll share with you that I had an
exhaust back pressure sensor fail as well and it had to be replaced. Don't remember
when this occurred if the RPM's were not going over 3,000, but, my vehicle didn't seem to
have the same power.
Few questions for you; is this the original engine and transmission? Have you ever
had the entire emissions system replaced? And, are you burning any excessive oil?
I am VERY IMPRESSED that you're almost at 500,000 miles, Congratulations again, I
want you to be my role model.