There are a few signs that can indicate if your car is polluting excessively: Dark or thick smoke coming from the exhaust pipe, especially if it has a strong, unpleasant odor. Soot or black residue on the rear bumper or around the exhaust pipe. Unusual or strong smells coming from the exhaust, such as a sulfuric or rotten egg smell. Decreased fuel efficiency, requiring more frequent refueling than usual. Increased emissions-related warning lights on the dashboard, such as the "Check Engine" light.