First, check the battery and starter cables for their tightness or free of corrosion and physical damaged.

See this " Starting system simplified diagram ". Have a help from an assistant to crank the engine, use a 12 volt light tester or a digital voltmeter, apply it to the starter control circuit.

If there is light or 12 volts and the starter isn't cranking, then the starter is the culprit. Have the starter to bench test for free at the local auto parts stores.

If there isn't no light or no volts, then the ignition switch is the culprit.

Check the starter relay and position the shifter at neutral.

http://www.samarins.com/glossary/starter.html

Loss of coolant because of a coolant leak is probably the most common cause of engine overheating. Possible leak points include hoses, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.

Make a careful visual inspection of the entire cooling system, and then PRESSURE TEST the cooling system and radiator cap. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A good system should hold 12 to 15 psi for 15 minutes or more with no loss in pressure. If it leaks pressure, there is an internal coolant leak (most likely a bad head gasket but possibly also a cracked cylinder or engine block).

It is important to pressure test the radiator cap, too, because a weak cap (or one with too low a pressure rating.

Heres a good write up on head gasket leaks and over heating

http://freecarsupport.com/.../...blown-head-gasket

Go to auto zone and buy coolant system cleaner! I wouldn't trust putting in amonia

www.freecarsupport.com

First step - have the check engine light scanned - for free -at any local auto parts store; Advance Auto, Auto Zone, NAPA - and post a reply as 'add answer' with the specific fault code for further assistance. It may be a question of compatibility with the aftermarket converter - but more detail will be available with the fault code.

02 sensor

Try a long needle nose plier.

Several possibilities; leaking valve cover gasket or gasket grommets, faulty / worn spark plug threads, loose spark plugs (not torqued correctly).

Check this sensor.
The Idle Air Control (IAC) valve controls the engine idle speed and dashpot functions. The valve is located on the side of the throttle body. This valve allows the necessary amount of air, as determined by the Powertrain Control Module (PCM) and controlled by a duty cycle signal, to bypass the throttle plate in order to maintain the proper idle speed.
Related Symptoms

Stalls after start when engine is cold
Stalls after start when engine is hot
Stalling on deceleration
Stalls at idle
Unstable idle
Diesels or runs on after the engine is shut off
High idle
Low idle

The check engine light doesn't have to be ON to store a fault code.

Check the vehicle for fault codes to see if there are any sensors were involved to the malfunction of the engine.

1.Local auto parts stores such as Autozone ( not in California) or Advance Auto parts will scan and retrieve any diagnostic trouble codes ( dtc ) stored in the powertrain control module for free of charge..

2. Amazon.com has the code reader for less than $20.00 dollars.

3.Re-post the dtc's in here " answer question " for further assistance.

Note: we need the dtc's to identify the sensor, there are a dozen different sensors to operate the car.

Visit this site and see all the different sensors.

http://www.freeautomechanic.com/car-engine-sensors.html

Navigate to this site and see the code reader

http://www.amazon.com/.../ref=sr_1_cc_1

Check these items first - before targeting the converter....

Possible Causes of Engine Hesitation or Stumble:

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)
Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

More than likely you dropped a valve seat