Q: I changed the spark plugs, now my car is shifting into reverse really hard, and it is stuck in 3rd gear

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I changed the spark plugs, now my car is stuck in 3rd gear and is shifting really hard into reverse. I didnt touch anything except the spark plugs and coil packs.

My car has 193000 miles.
My car has an automatic transmission.

What you are describing sounds like possibly low transmission fluid, a bad speed sensor or potentially a faulty transmission control module, which also controls the speed sensors. The transmission control module has various sensors such as vehicle speed sensors, wheel speed sensors, throttle position sensors, turbine speed sensor, and transmission fluid temperature sensors that all direct information to it in order for it to perform it’s function. Other functions controlled by the TCM are components such as a kick down switch, traction control system, and cruise control module. Any of these may also affect the way the transmission control module performs and controls the transmission and the shifting of your vehicle. A faulty transmission control module may cause also cause a delay in acceleration or erratic shifting of gears. I would suggest having a professional from Your Mechanic come to your location to properly diagnose your transmission problem.

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