this was a solution for my car a 2009 ss with only 19.993 miles.
this started almost instantly while waiting in line for gas at costco,
the car ran very rough and was getting around 7 mpg.

the codes were:
multiple
p0300 random misfire
p0301/02/03/04 misfire in cylinder.

the mechanic wrote:
monitor fuel trim,
long term fuel trim @30%,
short term fuel trim @28-30%,
lean condition,
ecm compensating by adding way too much fuel
causing cylinders to misfire.
after doing many tests at his shop and at a friends GM shop
a concensus was to try to unplug o2 sensor and the car ran smooth.
he then replaced o2 sensor bank 1 to correct condition.
now car runs like new.
i hope this helps with someone elses problem.
rich:

 

Welcome to the site! And thanks for the tip on the fix !

 

Great tip - and yup, welcome to the site.

 

Nice tip.

Yes, a belated Welcome (I see you actually joined this site 6 years ago ! )

 

Welcome!

There are 2 "bank 1" O2 sensors; which one? I assume #1, since it's the one that controls the fuel trim primarily. (There is only 1 bank on the HHR motor) they are usually referred to as B1S1 & B1S2.

Why didn't they just graph the O2 sensor outputs? That tells you immediately if one is broken, and even my $20 OBDII tool will do it.

Be glad that you got it fixed before the cat melted.