What will happen if the knock sensor fails?

Question

I have understood that most recent cars have a knock sensor. This allows among other things greater timing advance due to detecting knock in case the fuel doesn't work well with a greatly advanced timing.

Now, what will happen if the knock sensor fails? If it is unable to detect knock, is the engine at risk of damage from knocking? Or is the ECU smart enough to occasionally test a ridiculously advanced timing resulting in knock (which a correctly operating sensor would detect), and then detect a failed knock sensor and run with a safe timing resulting in reduced engine power output? Or is the ECU able to detect a failed knock sensor in some other way?

Are there any diagnostic trouble codes related to failed knock sensors? How often do knock sensors in general fail?

Answer 1

Most knock sensors in use today are microphones. The most common sensor type is a piezoelectric crystal type. Failure is detected by: 1 Testing the circuit during a power on self test. 2) watching for signal thresholds during normal engine knock producing operating conditions. Field testing is done with an oscilloscope and a hammer. A tap on the engine block should produce a voltage signal on a scope. Plus and minus 2 volts is common. Voltage varies with signal strength. P0325 is the standard failure code.
Engine damaging knock is quite loud and would be obvious to a driver. Almost always heard on acceleration. As usual OEM failure engine management strategies vary but the most common is to limit timing advance to a known safe range. This will limit both fuel mileage efficiency and power. Failure is common among some makes and less so with others. For example Subaru installed a high failure sensor from the mid 90's to about 2003. But Toyota sensor failure is uncommon.

Answer 2

They have been used since the 80's to maximize efficient use of fuel octane ( since the 70's for octane testing.) If they fail you lose some efficiency and power. I doubt failure will hurt the engine. I drove a 1964 Oldsmobile with 10.5 compression to the end using regular gasoline. It knocked so loud when accelerating, that other drivers turned to look. Yet a couple years later as I left it idling at junk yard , the employee couldn't believe it was so quiet. So extreme knocking had apparently caused no damage.