Its an 2000 Ford Mustang with an engine code

Not sure, but all OBD-1 codes are two digit codes. Check this site/page to see if you are doing it right: http://www.2carpros.com/articles/ford-lincoln-mercury-obd1-1995-and-earlier-diagnostic-trouble-code-definition-and-retrieval
Check the egr valve operation-may need cleaning the passage of carbon deposits.

if its a handchiffted gearbox put in highest gear.
puch the car to the front and back till you hear a click.
if he starts .check the ignition.if this is not correct the car have a death spot.

I'm afraid your right side Bank1 Converter is shot

420 & 430's are failed converters

It could possibly be a problem with your torque converter in your ******. Its definitly worth taking it to a transmission shop to get it checked out.

a haynes manual from a parts place will show you--or chec the condition of each plug wire-if one looks bad or falls apart--probly need to get all new wires /plugs

depending on the code that was cleared
you may have to complete 2 trip cycles to run all monitors
engine cold

this light has NOTHING TO DO WITH MAINTAINENCE, what it means is that a malfunction has occurred in the electronic fuel injection system or the engines electronic emission control system, there are well over 200 such faults that trun this light on,when the light is turned on a fault code is set in the memory of the computer that controls the system, to find out what this code may be you must have a shop test the system, this is called an OBD2 fault code test, you may also have it tested by any Kragen or Autozone Parts store for free in most area's. This willgive u a code but not any info oon how to fix the problem that turned on the check engine light, for that u need a qualified technician

ur engine is a v6, the head that has the number 1 cylinder is bank 1, so u need to go to the second head and follow the exhaust line, ull find a sensor there, probabbly has 4 cables looks a lot like a spark plug, thats ur oxygen sensor, that probabbly needs to be replaced.
hope this helps
have fun!

I replaced the coil pack to no avail. I also purchase a code reader while at the auto parts store. This device told me cylinder 3 was misfiring (P0303). I found that the spark plug wire had shorted out on the manifold sensor tubes. Replaced that wire and done.

I highly recommend keeping a code reader in your car tool kit. very useful.

On  4.6 engines the ECT is located at the left side of the alternator outside the engine block. Thsi si the one that the computer reads.