You are leaking (internally or external) the coolant. With out knowing the code that has set, it is impossible to say the cause of the light.
Something is affecting the emissions of the vehicle , it could be anything from the gas cap not installed properly , poor gas , a sensor . It could just be a temporary problem that corrects itself and the light goes out after using the vehicle a few times ( starting with a cold engine ) As long as the light isn't flashing which indicates an engine cylinder misfire , if the light stays on after using the vehicle a few times and trying some gas from another station I would take it in to a mechanic and they will scan the vehicle for any trouble code(s) to help diagnose and correct the problem
the engine light stays on. Hyundai dealer wants 241 plus to install and fix. Does using car with engine light on do damage?
A malfunction with the engines emission control system has been detected and requires service / repair . If the light stays on , and the problem doesn't correct itself after using the vehicle a few times , have it checked out . ( As long as the light isn't blinking , which indicates a engine cylinder misfire , and no other warning lights are on , and your engine temperature and oil pressure are O.K. etc. )
The check engine light cannot be retrieved on a 1990 Cavalier unless a problem develops with the engine. The engine can be checked for errors, though, by using a tester that is plugged into the vehicle's computer.
You didn't mention the year, make or model vehicle you have but, chances are it has a "check engine" light. If the "check engine" light does not illuminate while you're using the vehicle it is running efficiently as it is designed to run.
It means that a light bulb is out on the side where the light solid (not blinking) it could be in the front or rear of the vehicle.
The Service Engine or Check Engine light comes on when the ECU has detected a problem with the emissions system. Do not ignore this light even if the engine seems to be running fine. Failure to repair this can cause damage to the catalytic converter especially if the vehicle is not running good or using excess fuel.
There are a couple of ways to reset the Service Engine Soon light on a Suzuki Grand Vitara. The easiest way is to get a scan tool from your local auto parts store. Using the scan tool will reset the service engine light. Disconnecting your vehicle's battery is another way that often works to reset the Service Engine Soon light.
Using an octane rated lower than your vehicle specification will damage the engine. I would recommend not doing that.
The check engine light (service engine soon) comes on and stays on when a problem is detected by the self diagnosis system of your vehicle. Generally, the problem is in the emissions or something that is affecting the effectiveness of the emissions, but other causes can trigger the light to come on, too. To determine the cause, the vehicle will need to be taken to a parts store or a shop that has an OBD2 scanner, or for 1995 and earlier models, an OBD1 scanner, While a scanner will not give you the magic answer, it will give you a code that narrows down the problem area. Not using 91 octane should not cause a cam sensor issue.
using a code reader
Using contaminated fuel can cause an engine misfire which will turn on the check engine light. A faulty gas cap can for sure turn on the light.