Need a replacement catalytic converter for your 1991 Mercury Grand Marquis? MagnaFlow offers high-quality catalytic converters engineered for 1991 Mercury Grand Marquis sub-models and engine sizes. We combine efficient design and state-of-the-art manufacturing to meet or exceed required emissions standards. Select your 1991 Mercury Grand Marquis requirements and state vehicle registration to buy the best Grand Marquis catalytic converter online that fits your '91 Grand Marquis and meets your state’s emissions compliance.