1999 Nissan Pathfinder P0300 and P0325 codes. How to repair.

Question

Engine was hesitating. Mechanic said he couldn't find anything wrong but thought if a full tune-up was done would resolve the problem. I changed Distributor, Plugs and Wires. I had to remove the upper intake manifold to change 3 of the spark plugs. Engine is still running rough and now I have codes P0300 and P0325. I'm not sure what to do next.

Answer 1

How bad was hesitating?

P0325 code is for the knock sensor and can cause some minor, and sometimes hardly noticeable, hesitation, but the problem is probably the P0300 code.

The P0300 code means that you have multiple cylinder misfire. Since the problem started after you removed the the plenum and replaced the spark plugs, you should double check the spark plugs and plenum installation. Start by checking for intake air leaks by the plenum gasket.