The clean plugs were most likely in the affected cylinders. The coolant will clean the plugs and cylinder walls of carbon. The check engine light is on, because the car detected two spark plugs not running. A compression test will pinpoint the affected cylinders.
A head gasket repair on this car should be easier than a Supra. This is a simple 4-cyl in-line. I did one on my Toyota Pick-up in one day. I'm about to do one on a 1993 Celica.
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