cold 4L60E is not upshifting

[Taranaki]

I was test driving a VS II Berlina offered for sale by a pensioner - low milage, one owner. Only one problem.
When cold, the auto transmission is not shifting up until high revs. This lasts about a minute, then settles to normal shifting. ATF level seems OK. What's likely cause - dirty fluid, band brake, flakey electrical contacts?

 

[Brett_jjj]

It could just be a blocked up auto trans filter if it hasn't been serviced in a while... These filters are cheap and fairly easy to replace.. You could also check for any logged error codes by bridging pins 5 and 6 in the diagnostic connector under the steering wheel, some auto trans problems will log error codes to help diagnose whats wrong..