Trouble code PO300

What would cause a trouble code PO 300 on a 1998 Chevy Blazer. The truck will run good sometimes. When it runs bad it will stumble at low speed acceleration. It will run fine wide open. Dealer looked at it and thought it was egr vale. I replaced it and it runs a little better but still stumbles.

My first thought is try replacing the plug wires if they haven’t been replaced in a long time.

Since that is a random misfire code, I would wonder when the last time you changed the plugs and wires? If you have changed them did you use anything other than OEM plugs and wires? While you are at it, when was the last time you changed the air and fuel filters?

How old is the cap and rotor? Did the dealer check it? Did they test the ignition coil? Did they check the fuel pressure? Check for vacuum leaks?

You don’t need to take this to a dealer. Any decent independent mechanic can take care of this vehicle.

Does The Vehicle Have An Aftermarket Alarm System Installed ?


The misfires might be caused from a dirty/defective Mass Air Flow sensor.

Purchase an aerosol can of MAF sensor cleaner and try cleaning the MAF sensor. If this shows no improvement, then a scanner should be connected to the computer and then the vehicle taken for a drive. On the scanner observe the throttle position relative the mass air flow. So for example if at 100% wide open throttle the MAF sensor reads 80% mass air flow, the problem is with the MAF sensor.