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Bad speed sensor?

My moms '94 Mazda 626 is acting strange and I wonder if a bad speed sensor may be to blame. The trans is shifting weird( sometimes slips when going into 3rd gear and has to be shifted manually). A bad tranny was ruled out because it’s been doing this for over a year. Now it’s started to idle down and cut off.

Could it be the speed sensor? If so, where is it located and is there any way to test it?

BTW-The air filter housing has a pretty bad crack in it. I mention this because I noticed it won’t even start unless the housing is clamped down tight and I’m not sure if there’s some sensor inside not liking the extra air that could be affecting performance.

Thanks guys.

I’ll assume that it’s an automatic transmission, and that the engine is a 2.0L four cylinder. Is this correct?
If it’s an A/T, have you checked the fluid level? Have you replaced the A/T fluid and filter?
You need the repair manual…Haynes, or Chilton’s. It may tell you how to check the Turbine Shaft Speed Sensor (or, it may be called a Vehicle Speed Sensor (VSS). It’s on the transmission. See for some free info.
Yes, cracks in the plastic air intake housing and duct can adversely affect how the engine run. Replace, or duct tape, any cracks.