GM Repair Problems? Ask GM Repair Expert Now.
P0300 is a multi or random misfire code and might be caused by one of the other codes. The P0442 could simply be a loose gas cap, so tighten the cap and clear the codes and see what comes back. The P0140 could be an exhaust leak. Place your foot over the tailpipe while the engine is idling and listen if the exhaust noise gets louder or if you hear any hissing. Then run your hand down around the exhaust manifold gasket where it meets the pipe and then along the exhaust manifold to head feeling for any hot exhaust gas blowing on your hand. If so, you have an exhaust leak.
Once you find and fix the causes of the codes P0442 and P0140, clear the codes and see if P0300 comes back and/or if the engine still misfires and/or runs poorly. If it does, try a tuneup, including spark plugs.
Here are the possible causes though of both codes:
Charcoal canister is loaded with fuel or moisture
Fuel filler cap is loose, cross-threaded, damaged or wrong part
Fuel tank, fuel filler neck or fuel sending unit 'O' ring is leaking
Fuel tank pressure sensor is damaged, disconnected or it failed
Fuel tank vapor line(s) is clogged, damaged or disconnected
Purge valve vapor line is clogged, damaged, or disconnected
Purge solenoid or Vent solenoid has a small leaking (sticking)
PCM has failed
Exhaust leak present in the exhaust manifold or exhaust pipes
HO2S signal or ground circuit has a high resistance condition
HO2S element is silicon, water or fuel contaminated
Base engine mechanical fault that affects one or more cylinders
Fuel metering fault (high fuel pressure or fuel contaminated)
EVAP system problem or the EVAP canister is fuel saturated
EGR valve is stuck open or the PCV system has a vacuum leak
Ignition system fault (a coil) that affects more than one cylinder
MAF sensor contamination (it can cause a very lean condition)
TSB 99-06-04-005B contains a repair procedure for this code
TSB 03-06-04-030 contains a repair procedure for this code
Here is that one tsb that says it may be fuel injectors need cleaning,
I did find a reference though to 99-06-04-005B SEP 01, Fuel Injectors - Hard Starts/Poor Driveability/Misfire so it's probably the same thing and fuel injector cleaning.
Now I hear Walmart stores with a tire shop will clean fuel injectors for $20, but I have not actually confirmed this.