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Lexus knock sensor

Loss of power, check engine light on, overdrive inop. Dealer says replace knock sensors (2), harness, etc for $1100. Parts not in stock. While traveling to dealer with parts, and refueling, engine ran good, light went out, and overdrive OK. Have driven car for six days with no problems. My questions are:
Does the engine light indicate the knock sensor activated as it was supposed to, or is it truly defective?
How is the engine light able to reset itself?
Why are there two sensors?
If only one is defective, wht replace two?

The ECU does have codes for a defective knock sensor, but without knowing what codes the dealer found and what validation tests he did, it’s impossible to say. Realize that a knock sensor detects preignition, so the problem could be preignition rather than a defective knock sensor. Can’t tell anything from here, especially without the codes.

The ECU can for some problems reset itself after detecting a fault once cycled no & off a certain number of times. It may be that whatever caused the preignition has not recurred. Could be that you drive up a high hill trying to accelerate more than normal and a weakness in the EGR system (what year IS this Lexus, by the way? What model? What engine?) allowed the cylinder temp to get too high and preignition occurred and you haven’t gone up that hill since. No way to guess from here.

There is one sensor for each cylinder bank. You have a V6. The knock sensor is a crystal attuned to the engineering-determined frequency that preignition causes, sending out a signal to the ECU when that frequency is detected. The sensor is mounted on the head, through which the frequency travels. One on each head detects the impulse no matter which bank of cylinders it occurs in and also allows the ECU to tell you which bank had the problem via the fault code. There’ll be one code for “bank 1” and another for “bank 2”.

Honestly, unless this a warranty work I think you’d be much better served by an independent shop. Dealers in my opinion don’t want to bother to due true diagnosis. They just guess at parts to replace based upon the codes, all at your cost. This forum is replete with posts about problems getting cars repaired at dealers.

By the way, how many miles has it been since you last changed plugs? If it were my car, I think I’d replace the plugs and filters and drive on unless the code recurred.

Model-Year ?
Approximate Miles ?

Procrastination: I have successfully repaired cars before using the Procrastination Method, especially repairs in the $1000+ area.
Just One Example: My daughter’s car had CEL on and a code for a fuel tank pressure sensor, I believe it was. I bought a sensor, turned out the light, cleared the code (both events) and had her keep driving. Never had another hint of a problem, before or after the incident. Problem fixed by doing nothing, I returned the part 2 months later.

I’d give it a while longer and see if the problem comes back. Spend $50 on a code reader (Actron Pocket-Scan from Advance Auto Parts would work) and you can check your own codes and turn the CEL off, yourself, OR Advance will do it with their’s for free.

See what happens. If the CEL is Flashing do not drive the car. That could cause damage.


Did you purchase your gasoline recently at a gas station you normally don’t use? It’s possible this is a fuel quality problem, and you’ve used enough of the bad gas that the problem has gone away. Just a guess, but worth considering, especially if the problem went away soon after re-filling at a station you normally use.

Thanks for the comments. I was driving from Nashville to Houston. Problem occured near Huntsville, Al. The car is a 1999 RX300 with 180K miles, well maintained. I believe the problem was caused by the gas. Will the sensor code remain stored in the CEL? I don’t know the code numbers.

It sounds like it cleared, but it can be easily checked. Lots of parts stores will check for free.
Just a detail, it gets stored in the ECU. The CEL is only the light that tells you the ECU has detected a faulty signal from one of its sensors. The CEL can be off and the ECU still have a retained fault code.

It could have been the gas. Gas that detonates too readily (lower octane than the pump label claims) can manifest itself as preignition, and that would be sensed by the knock sensor.