My son was back from college which is a 400 mile each way trip. I added a half quart to the 5 quart sump and the other half to a 20 gallon fuel tank. I drove the car approximately 80 miles around town before he headed back yesterday so basically 500 miles on the Kreen. He called last night and said as he was getting home, the check engine light came on. Also said he started the car a little while later and it hesitated but has ran fine all day today. I'm obviously gonna have him go to auto zone and get i it scanned. The cars used oil but never in 111k threw a cel. Anyone have a Kreen related cel or other problem after using it I should be aware of?