Engine hard start

We need the number, not their interpretation of it.

Before you do anything like replace parts, scan for codes and check fuel pressure with a gauge auto parts rent it. It may not even be the O2 sensors. Fuel pressure should be 60-66 psi with key on

If it's a tbi unit check underneath with choke cleaner for a blown gasket. If not check fuel pressure with a gauge auto parts rent them

Check the fuel pressure if okay test the fuel pump relay-both okay test the coolant temperature sensor

GM has a ton of trouble with their pressure regulators but they typically leak fuel into the vacuum hose. To tell if it is not sealing well and is letting the pressure bleed down, use a hose pinch-off pliers to pinch the rubber hose where it goes from the fuel rail to the body. To check for a leaking injector you'll have to pull them all out still attached to the fuel rail, then watch to see if any get wet on the tips when you turn the ignition switch to "run". A less common problem is a leaking check valve in the fuel pump. You can identify that by pinching off the fuel supply hose right after a helper turns the engine off. GM also has a lot of trouble with misfires caused by mismatched injectors. They just grab six or eight of them from a box without matching their flow rates. You can find rebuilt injectors for a lot less money and they will be packaged as a matched set.

Clean out the idle air control valve and test the throttle position sensor/manifold absolute pressure sensor.

Wouldnt that cause problems all the time if those sensors were dirty or bad

It will and may and can set off the check engine light-I strongly recommend you check and test it-to include a fuel pressure check and get back with the readings for evaluation and also test the fuel pump relay and the fuel pump and oil pressure sending unit

Does the wait to start lamp inthe dash come on when you turn on the key?