If you install your MegaSquirt system in “parallel” with your stock PCM, a check engine light is unavoidable. If you replace your stock PCM with your MegaSquirt system, you probably won’t physically see the check engine light. In either case, your car will probably not pass an emissions test. MegaSquirt ECU’s are not meant for use on any pollution controlled vehicle. There are strict penalties for tampering with or disabling the stock pollution control devices on cars in the United States and Canada.