Running rough until warm

The code 34 is the MAP Sensor and the code 44 is the O2 sensor reading lean.

It sounds to me as though the MAP sensor is telling the computer to cause less fuel to be injected into the engine. I would replace the MAP sensor first or clean it, and then reset the computer and see if it still throws the code.

Ok. Verify that both are the correct part numbers for your truck. Make sure the connections are good and solid. Make sure the MAP sensor is properly installed. Make sure the wiring between the MAP sensor and the computer and the O2 sensor and the computer is good.

Also, have you reset the computer since replacing these parts?

What model Chevy is it?

Our reference only lists a MAP sensor for your vehicle. We use Mitchell, which compiles information from the manufacturer.

Do a compression test. Check fuel pressure. I would also run some fuel system cleaner through it. Also check for vacuum leaks.

You're very welcome. Let us know what happens.

That sounds to me like it's either a wiring or computer problem. Or maybe a faulty O2 sensor. Even new ones sometimes have problems.

Compression is the amount of pressure built up in the cylinders when the piston moves up. To test it you remove a spark plug and crank the engine over. I also like to disconnect the coil to keep the engine from starting. You have to test each spark plug/cylinder. You can borrow a compression tester from places like Autozone and Advanced.

Look for what looks like a tire valve near the throttle body/fuel rail. That's where you check fuel pressure. You should carefully bleed some of the residual pressure off (like throw a towel over the valve first to prevent spray all over) before hooking the gauge up.

Were you able to find a compression tester?