Engine Performance problem
1996 Ford Contour 4 cyl Front Wheel Drive Manual 150000 miles

I've got a 96 contour that will hesitate when you press the accelerator until it gets warmed up, then runs fine.
The check engine light was on and the codes are:
P0420: Catalyst efficiency below threshold (bank 1)
P0171: System too lean (bank 1)

I've cleaned the MAF, throttle body, and checked the vacuum lines, but problem still persists. The car has recently had plugs and wires changed also. Is there a common problem where these two codes come up? I don't want to spend a whole lot of money throwing parts at this problem, and I'm thinking it's the catalytic converter, but I don't want to spend that much money unless I know that this is the most likely culprit so any help is much appreciated. Thanks.

P0420: Catalyst efficiency below threshold (bank 1)

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for
An oxygen sensor is not reading (functioning) properly
The engine coolant temperature sensor is not working properly
Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
Retarded spark timing
The oxygen sensors in front and behind the converter are reporting too similar of readings
Possible SolutionsSome suggested steps for troubleshooting a P0420 code include:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
Inspect the downstream heated oxygen sensor (HO2), replace if necessary



P0171 A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)