Owner of this 2005 Cobalt decided to install a smaller supercharger pulley to increase boost to 18 PSI, installed larger fuel injectors and added water/methanol injection.

He finds the car is running very poorly and wanted us to custom tune. Existing tune was GM's stage 2 for the supercharged engine so we doubted the base tune would be real far off to cause the problems we saw

We had him do a testrun with Ease OBD scanner and send us the recording to find.

Average part throttle fuel trims 19% lean yet very little engine knock yet lots of misfires.
Maximum fuel injector duty cyle at 174% !
Front 02 sensor reporting less then 300 mVolt average while rear 02 staying around 960 mVolt.
PCM was tripping high voltage P0103 DTC and P0171 lean DTC.

In the scanner data we analyzed the finding was in fact the engine was really running super rich as the clue was rear 02, no ping yet misfires so we asked owner to pull all 4 sparkplugs and both 02 sensors to find in fact the plugs were black and the front 02 sensor's tip had broken off.

We'll post photos and results of our custom tuning once the plugs and 02s are replaced but again shows a OBD scanner and knowledge on how to use it and what all the data means can pinpoint issues you just do not think was the cause.

Images of both 02 sensors showing how front 02 was damaged and how really rich the AFR was that fouled up the new sparkplugs

Now with the 02 failures fixed and a exhaust leak found also fixed we see the fuel trims are no longer super lean and with base tune flashed in everything looks fine except the fuel injectors are too small for 18 PSI of boost at a 118 % duty cycle of injectors

This is another example of a car being 2,500 miles away but the proper use of a OBD scanner allowing me to find problems and tune for better performance without touching the car myself