First, a couple sentences on just what "knock" is. In your engine, the fuel mixture is injected, and then the piston begins its compression stroke. Somewhere just before the piston reaches the top of its travel, the spark plug fires, igniting the mixture. That's actually a well-controlled burn and it creates the pressure needed to drive the piston down through its power stroke. "Knock", also called pinging, occurs when something causes the fuel to ignite in an uncontrolled manner, usually before the spark plug even fires. This is a violent event in the engine - heavy pinging can lead to all kinds of nasty internal engine damage.

Remember the cars of the late 70's and early 80's? It sure wasn't unusual to hear them knock. But in today's vehicles you (almost) never hear as much as the slightest knock. Improved electronic controls play a big part in that, but the unseen hero is our friend the knock sensor. This little guy detects vibration - but much more than that, it can actually tell the difference between the engine's normal shakes and rattles and engine-damaging knock. You'll find the knock sensor mounted somewhere on the engine block or intake manifold. Some engines even use more than one.

What are the symptoms of a failed knock sensor? It can be pretty subtle and not that noticeable in every day driving. There are two things that can happen. First, the knock sensor could fail to detect knock leading to audible pinging when conditions are right. If you hear pinging you definitely need to check out the knock sensor. Second, the knock sensor could be overactive - detecting knock that's not really present. That translates into reduced power and economy.

How do you test a knock sensor? Pretty simple really. Plug in a scanner,then take your favorite wrench and give the engine block a couple raps next to the knock sensor (please, don't hit the sensor itself!). Watch the ignition timing and knock sensor parameters onthe scanner- you'll see knock events register on the knock sensor parameter, and you'll see the ignition timing retard a few degrees. If you don't see any change, be suspicious of the knock sensor and its wiring harness.
Another possible problem with knock sensors is mis-triggering on some other vibration. One example where this is common is on modified cars with aftermarket headers. Space is usually pretty tight, and a header tube might rattle against another part under acceleration, or a manifold bolt might work loose. These subtle problems can drive knock sensors nuts. You might also see some knock sensor activity on a badly worn engine from a noisy valve train or worn bearings.

A misbehaving knock sensor has a dramatic impact on performance and gas mileage and its one of the first things you should check out with your scanner. Turn on the data logging feature and go for a drive. With a warm engine, head up a couple hills at highway speed. Check the scanner for knock sensor activity. If you don't see any, and don't hear any pinging, you can rest easy that your knock sensor is doing its job and not robbing power or mileage from your engine.