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hey, friends got a 4cyl auto. it shifts up into second no problem but it kicks in and out trying to go into third but it won't go. no flashing hold light either...the light does work. oil is clean and does not smell burnt. is there something wrong with the tcu or dirty sensor? any idea's. thanks guys.

it can be a few things causing it here are the causes;
1) worn out clutch packs causing slippage (usually the oil will be darker coloured or contaminated if this is the case)
2) neutral safety switch, all the wiring for the shift solenoids is routed through this switch and it wears out. To test try to start the car in gears other than P or N if it does then that is your problem
3) Plugs in between the trans and ecm are corroded causing lost signals. Check all plugs for corrosion and correct as necessary.
4) Low atf level, check it with the car fully warmed up and running in P.

In addition to these problems also check the engine ground strap and the transmission ground strap. Check the transmission kickdown cable see if it has a lot of slack this cable stretches a little over time and can cause shifting issues, correct this by marking the cables original position and move it away from the thorttle to increase transmission pressure but do so in small increments or with a pressure gauge good for 700 psi attached to the trans. If the trans is being subjected to a lot of stop and go traffic add an aftermarket cooler and external filter to it to upgrade the cooling thereby allowing for a much longer lasting transmission.