Q: Car misfires; crack in manifold

Hello. A crack in the exhaust manifold near the O2 sensor can cause some poor sensor readings to the computer. The crack will allow fresh air to enter the exhaust, which causes the sensor to tell the computer that there is too much oxygen. This will send a message the computer to add more fuel, thus causing misfires. If the engine is running rough and misfiring without any actual misfire codes, then the crack may be the problem and the manifold will need to be replaced by a trained technician. A mechanic would typically scan the computer for any codes and see what the fuel trim readings are in order to be able to verify that the crack is the issue. If you do not have the tools to do this or don’t feel comfortable doing this type of repair, I would recommend the assistance of a certified mechanic that can replace any components causing problems in your vehicle.