I have a 2007 Chevy HHR,Has auto trans and won't go into gear untl it warms up -3min. Roughly.Engine seems to be racing up to that point. If I start it and shift it -right away it jumps into gear.After the 3 min--it works great all day even when left for hrs.Doesn't seem to be a mechanical problem but a bad Sensor?

I have a 2007 Chevy HHR,Has auto trans and won't go into gear untl it warms up -3min. roughly.Engine seems to be racing up to that point. If I start it and shift it -right away it jumps into gear.After the 3 min--it works great all day even when left for hrs.Doesn't seem to be a mechanical problem but a bad Sensor??Has anyone pulled the computer codes yet to see what is in there... and there might be something. Particularly if it's a sensor.

Check for codes. It could be a bad transmission speed sensor. Transmission speed sensors report transmission output speed to the PCM. When not replaced, the transmission may not shift out of low and it may experience inaccurate shift points.

Has anyone pulled the computer codes yet to see what is in there....and there might be something. Particularly if its a sensor.

There hasn't been an engine light yet,so how could I pull a code

Check for codes. It could be a bad transmission speed sensor. Transmission speed sensors report transmission output speed to the PCM. When not replaced, the transmission may not shift out of low and it may experience inaccurate shift points.

There hasn't been an engine light,otherwise how would I pull a code

Sometimes, "soft codes" can be read, which haven't created a hard fault--it says there is nothing wrong now, but there was some error in the past. They are there to give the tech a "heads up."

You need someone with a code reader, you have an ODB2 car and there will likely be codes stored there. On these vehicles they can be very comprehensive.
While I'm not familiar with the HHR... I do know many vehicles the last 20 years use electrical solenoids in the valve bodies that control the shifting... they can get sticky but should register on the computer when they aren't responding correctly. Particularly GM... And I know Saturn did this in 1996, as I had experienced something remotely similar (but it was reverse in my case).

A code will give a professional a better idea where to look, saving time, and time is money, as he won't have to spend time narrowing it down to a subsystem. Particularly with a problem that's intermittant.