Q – My 2003 Carrera recently began to run rough, the Check Engine light came on and started flashing. I shut it off and when I started it back up, it ran fine again but now the light is on all the time. I wonder what might be causing this and if it is safe to drive with the light on? –Sam

A – The check engine light indicates critical issues by flashing. When the engine started running rough it was probably misfiring. You were smart to turn it off and restart it. Doing so, reset the computer misfire detection. Driving after you restarted the car is not an immediate problem although eventually the misfire will come back. When the light comes on, if the engine feels strong and runs OK, you can drive the car to a shop to have it scanned and diagnosed. I suspect that when this occurred it was cold or wet. Don’t be surprised if it happens again during torrential April showers.

Driving with a misfire may do some damage, so when the computer detects a miss it will temporarily stop the fuel injector pulse at only that cylinder. This protects the cylinder from getting washed clean of oil by the fuel. It also prevents raw fuel from entering the catalytic converter which can cause the cat to overheat.

On newer models, there are two common problems that I find. One is oil-soaked spark plug coil packs caused by a failure of the spark plug tube seals. The other is failure of the coil pack itself caused by a faulty manufacturing process which leads to cracked insulation. On many of the coil packs (there are six per engine) the steel component is exposed to humidity which causes them to rust, expand, and crack. Fortunately, they are not too expensive to replace. When the coil packs are replaced, I suggest replacement of the spark plugs as well since they are also not expensive and easily accessible with the coil packs out.

Incidentally, on older pre-OBD (on-board-diagnostics) cars—the ones without a check engine light—the computer will not shut the fuel injectors off! The effect can be much more dramatic with a great likelihood of expensive damage. On these cars, driving with a misfire can only make it worse. It is best to tow the car rather than drive it. With the older cars, replacement of the spark plugs is essential. Experience has shown that even if the misfire is cured, a previously fouled spark plug will usually re-foul.   –MC