Toyota Repair: knock sensor 1, shadetree mechanic, octane fuel

Question
QUESTION: On my 2000 Prerunner, 2.7 4 cylinder I have an error that seems to reoccur when ever I go to the mountains.  I currently live at roughly 70 ft above sea level.  This only occurs when I go to my home town at about 4500 fft above sea level and stay for a week.  It comes on after a few days and stays on for a seek or so when I get back.  It usually causes the check engine light to stay on and will cut off after about a week.  It usually cuts on and off several times before it goes off for good.  I can seem to even find the sensor and now that it is off again don't know if I need to replace it.  Help.

ANSWER: This probably nothing to worry about, have you tried a higher octane fuel when going to the higher elevation? Do you know what the trouble code number was that made the check engine light turn on?

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QUESTION: Yes, I tried mid and high octane but when the check engine light is off, there is no knocking.  The error code is P0325.  120,000 miles and no other problems.  When light is on, the engine is sluggish when accelerating and if I push it too hard, it will knock but stop if I back off the gas.  Almost like I'm in too high of a gear.  The light has gone off a few times for a couple of days but is back on now.  Is this sensor something a shadetree mechanic can change?  Looking on the Internet, I see that the sensor may be under the intake which means I would have to take the top of the engine off or would it be better left to the dealership?  Approximate cost??

Answer
The knock sensor can be replaced by removing the starter which makes it a lot easier to get to but I'm not sure replacing it will solve the problem since it is reacting to an engine preignition condition, this causes the knock sensor to send a signal to the ECM to retard the timing, this explains the loss of power. The light will eventually turn off because it is designed to do that after the problem corrects itself and the computer sees the correct signals after so many trips. The engine pinging at higher altitudes could be caused  by carbon builtup on the pistons which can raise compression, in that case using a higher octane fuel should at least help somewhat. One other idea I have is that the engine may be running a little hotter at the higher altitude, try replacing the thermostat and see if it helps or find the cause for the overheating, maybe the cooling system is not as efficient as it used to be and the radiator may need to be replaced. The bottom line is that engine preignition is normally caused by excessively high combustion chamber temperatures or timing that is too far advanced, timing is controlled by the computer and can't be changed, the computer itself may be the problem but it's not likely.