Toyota corolla 2000 engine light comes up frequently

I have a toyota corolla 2000, with 60,000 miles on it. Engine check has started to come on frequently for the last two months. I went to the dealer and he said he fixed air vent now eveything should be ok. But two days engine check light is on again, now the dealer says he cant say for sure what is wrong. He says 1) Run cans of BG44k to cleanup fuel injectors ($50) if that doesnt fix 2) Change flat lined 02 sensor ($268) and if that doest work 2) clean injectors and pressure test. All options are expensive and iam looking for soem expert advice.

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