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2000 chevy suburban

Vehicle misfires. Autozone says it is random cylinder misfire code po300 and fuel pressure high or low / fuel circuitry high. I changed the spark plugs and spark plug wires checked good. The problem seems to be getting worse. What could possibly be the problem.

i have a 99 suburban with the same problem it turned out to be the mass air flow sensor even though the code read random cylinder misfire. i also had my catylic converter changed as well in case there were any blockage in there. all in all it seemed to help

When I had s similar problem on my '95 it was the spark plug wires shorting out. How did they check the wires?