[bwdz]

The freeze frame is an OBD function. It records data when the code is set. It can be accessed with even cheap code readers. It will show you the snapshot of the sensor data at the moment the code set. It will typically show the baro, o2 readings, ect, vehicle sensor speed, load etc.. etc... If there are multiple codes the freeze frame function will only remember one but sometimes up to 3 freeze frames on newer cars. It will keep the frame from what is considered the most important dtc. Essentially what you want to do is drive around and gather data to see what sets the code but your computer already has a snapshot of all the sensor data at the moment it threw the code. It is as if you took a picture of the sensor data feed the moment it decides to throw the code. There are possible clues in there.
If you are suspecting increased compression from the head replacement there is also a way to use a starting system oscilloscope that most garages have to look for clues. Disable the fuel and spark and hook it up. Crank the engine over. The wave form of starter current draw spikes on each compression stroke of each piston. If there are different heights in the spikes on 3 of the cylinders you will see that and by following the firing order you can make a guess that it relates to the 4 5 and 6. Essentially you are using the power that the starter must use as a compression tester. I use this to look for potential misfires when I suspect internal engine problems as it can hooked up in under a minute and requires no removal of anything on the car but a relay or fuse and it can find me a dead cylinder in seconds rather than pulling parts off a car.
I recently found a problem on a supercharged Cobalt in my shop using the freeze frame. The car had been to multiple shops and dealers and had the problem for over a year. It would run great but once a week or so would pop into limp mode for map correlation. The car was highly modified with different injectors and small sc pulley. Everytime someone diagnose it everything was fine, it would test drive fine once code was reset and all sensors were in spec. The freeze frame showed 0 volts on the SCIP sensor. The SCIP and Map were replaced several times by different shops and the car would run great for a few days and then the code would come back. By seeing the freeze frame with 0 volts I was able to determine that there must be an intermittent connection and sure enough if you wiggled the plug hard enough there was, so it worked 99.9% of the time but the freeze frame showed that it must have a bad connection.