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sounds good starts right up,drives in drive but wont upshift to 1.2.or 3,new shfit cable.new neutral switch,rebuilt tranny.new fluid and filter.02 senser are working,speed senser is working,i feel its possibly sfift soiloid or 2-1,3-2,4-3 soiloid,presure soiloid can the scan tool check those,pump is working  cause it drive in drive,shows no codes
Vehicle: 99 Chevrolet tahoe 5.7

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The ECM what it does is look for open circuit , short circuit, rationality, The main purpose for it is to protect the catalytic convertor, This is the reason for the CK ENG LT program. The catalytic convertor protects the envireoment from harmful emission emitted into the atmosphere, If you have a transmission fault you would set a P0700 or P07XXX codes these are for any transmission problems. What you have on this engine for the transmission downshift is a TV cable or the downshift cable, these are adjustable, if it is out of adjustment the downshift is delayed, (you will have downshift regardless due to the manual valve) . If you have a functioning downshift cable and a downshift,when you need it and still no power, you would need to look for another cause of no power, Any recent repairs made to the harmonic balncer or any front engine seal work

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