The technician determined the cause of a check engine light being a bad spark plug. They took several steps to verify this including moving the plug to another cylinder and testing for the issue to follow, which it did, and test driving the truck after replacing the plug to monitor for additional warning signals. Furthermore, they noted the plugs and wires had just been replaced last year and were under warranty so there was no charge. This is typical of the level and quality of the service the we have received at Oswald's over the years - thorough and professional.