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Q: knock sensor

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mechcanic changed harmonic balancer and now check engine light came on and indicates knock sensor

My car has 165000 miles.
My car has an automatic transmission.

Hello. To understand what is going on, lets discuss these two components and how they work. Harmonic balancers are essentially vibration dampeners in the engine that counteract the torque caused vibrations coming from the crankshaft. Every time the combustion cycle takes place this torque vibration is amplified a little, then when you have thousands of cycles going per minute, you can image how this vibration can add up. Now on the other hand, we have knock sensors which are supposed to sense any extra knock caused by spark timing. If extra knock is sensed, then the computer retards the spark voltage and attempts to compensate to help preserve the engine. So, if the harmonic balancer is loose, installed incorrectly, or faulty to begin with the crankshafts vibrations are not being dampened. These vibrations are what is causing the issue with the knock sensors. They are not able to solve the issue (because it is unrelated to their system), and are telling you that there is something wrong. Have the harmonic balancer tested and reinstalled as needed to make sure it is functioning as needed.

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