ECM-4201 is stored if the ECM registers that the rear heated oxygen sensor is switching too quickly.

Possible problems are:
1. Air leakage in the exhaust system.
2. One or more damaged injectors.
3. Damaged catalytic converter.

To check for 1 you should have an exhaust pressure regulator which builds up the pressure in the system. The joints in the exhaust between the cylinder head and the rear oxygen sensor are then sprayed with a soap solution to find out if there is a leak. Or, with a cold exhaust and a helper, start the car and stuff a rag in the exhaust pipe and listen for leaks between the manifold and O2 sensor.

To check for 2 you should probably remove them and have them professionally cleaned.

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My advice is free, so you got what you paid for...