Dear Tom and Ray:
I have a 1999 Subaru Outback that I love very dearly. Last week, the engine light came on, so I took it to my very reputable mechanic. He referred me to the dealer, because my "knock sensor" was off. He warned me that this would be expensive to fix. Since I am quite poor, I am wondering if this is a critical repair. What is a knock sensor? Thanks. -- Edith
RAY: The knock sensor, as the name strongly implies, senses when there's a knock occurring in your engine.
TOM: And it signals the computer to yell, "Come in!"
RAY: That's awful. Engine knock is also known as pre-ignition. It's when the gasoline in a cylinder detonates before it's supposed to. It can happen if you have fuel with too low an octane rating, if the engine is running hot or the outside temperature is very high, or if the engine is traveling under a heavy load -- like climbing a hill or accelerating hard with a couple of mothers-in-law in the back.
TOM: Pre-ignition is bad for the engine. If those extra explosions are allowed to continue over time, they can do damage. The knock sensor alerts the engine-management computer that knock is occurring, and the computer then retards the ignition timing to correct for it.
RAY: So the answer is that you should fix this, especially since you love the car and presumably plan to keep it for the long haul. It's not an emergency. It's not something you need to rush out and do today, Edith, but it's something you should fix as soon as you can.
TOM: The other reason to fix it is that, until you do fix it, your "check engine" light will stay on. You might say: "Big deal. I don't care if that little light stays on." But here's the problem: If the engine develops ANOTHER malfunction, you'll never know, because your engine warning light is already on. You'll assume it's on because of the knock sensor, and you might miss something important.
RAY: So I'd get it fixed, Edith. It should cost you about $200. That's not cheap, but these sorts of repairs are the cost of owning a car, and you have to expect them occasionally.