I have a '98 v70 AWD with about 117K miles. The OBD code indicates a fault with the rear knock sensor. My mechanic reset the computer and suggested I try using higher octane gas, which I have been using for the last 4 months or so. The code came up recently.

My question is this: Should I replace the knock sensor, or should I just continue to run higher-octane gas and not worry about it. Also, is this a do-it-yourself job? (I cannot find any information on this on Bay 13.

There are 2 knock sensors, both should be replaced at the same time because the wiring harness has been upgraded. The sensors are on the block under the intake manifold. A good time to replace oil seperator and vacuum lines while there. It takes 5+ hours for DIY.

If you want, I can send you a PDF. Also check www.matthewsvolvosite.com/index.php

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My name is Klaus and I am a V ♂ lv ♂ holic.

Wow. After being duly intimidated by the info on Matthew's Volvo Site, I'm deciding that this is beyond my skill level. My next question becomes, how important is it to replace malfunctioning knock sensors? Do I risk engine damage even if I am careful to use good gas, continue to keep up with regular maintenance, etc?

Thanks again!

The computer uses the knock sensor to regulate timing and amount of fuel being injected. You won't harm the engine, unless you do a lot of hilly driving in the summer or pull a trailer with 87 octane gasoline.

When you hear the engine pinging, back off the throttle.

One of these days, the oil separator will need to be replaced and then replace the knock sensors also. It is not as hard as the pictures describe, just a couple of bolts that are hard to reach. The injector rail does not need to be removed.
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My name is Klaus and I am a V ♂ lv ♂ holic.

Thanks again! How will I know when the oil separator needs replacing?

It all depends on the way the car is used and maintained, usually at about the 150K mark. Those that develope vacuum leaks and need to replace the lines do it then.
If the engine is always bathed in synthetic, it usually lasts longer. Same is also true for those who rev the engine higher, getting the oil hot. Frequent short trips with the outside temp below 32F cause the separator box to clog.

In short, there is no way to define when the box is getting close to being clogged, or the passage from the block to the box.

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My name is Klaus and I am a V ♂ lv ♂ holic.