Help with code 172

Sounds like your mechanic is on the right track. The fuel pressure is regulated by manifold vacuum, so a decent leak will deliver the wrong vacuum. However, that would normally give you lean/rich on both sides, not just one. It's worth a check anyway.You can hook up a fuel pressure gauge to the schraeder valve on the fuel rail near the oil fill cap. You should have a constant ~33PSI at idle. The fuel pressure regulator is on the back corner of the fuel rail with a vacuum line leading to it. While at idle, pull off the vacuum hose (plug it with your finger). The vacuum should be ~38PSI.

If I remember right, low values usually mean a bad fuel pressure regulator. Fluctuating values usually mean a bad fuel pump.

The fuel rail in theory should have even fuel pressure on both sides. Since our fuel system is a "return" fuel system (unused fuel is routed back to the tank), I would think that a blockage or restriction in the fuel rail would affect both sides. You could possibly have clogged fuel injector(s) on the driver's side, meaning that the fuel from the rail isn't getting into the cylinders.

You also could have an exhaust leak on the driver's side. If the exhaust is vented before it gets to the O2 sensor, the computer will read lean. Might be worth checking if your exhaust manifold bolts are tight.