When the light is on - take it to an AutoZone and have them read the codes.
Then you will have an idea of what needs attention. Anything else is
speculation. Are there any other symptoms/irregularities like smoke, hard
start, hi temps, etc?
Your check engine light comes on because the knock sensor is detecting the
pinging and even retarding the timing doesn't help. The compression in your
cylinders is too high. I am sure you just have to clean those carbon
deposits out. There are many products out there you can use. Just go to the
autoparts store and ask to the (hopefully) knowledgable person behind the
counter about your problem, they should be able to get you a good carbon
cleaner (with instructions) for you engine.
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