P0204 = Injector Circuit Malfunction - Cylinder 4
You will need electrical repair training and tools: volt-ohm meter, scan tool (for freeze frame data), noid light, wiring diagram / schematic, repair manual.
Plus the skills and training in wire harness and connector testing (pinout / wringing / continuity)
1) Learn the basic electrical theory, system and knowledge (ohms law for example).
2) Start with these components - harness, injector, connection, ECM
What causes the P0204 code?
Open or shorted wiring harness,
Failing cylinder 4 injector,
Poor electrical connection,
ECM is faulty.
Diagnose the issue in the order shown.
How does a mechanic diagnose the P0204 code?
Hooks up a scan tool into the diagnostic link connector (DLC) port on the car and pulls all codes that are stored in the system
Observes the freeze frame data associated with the codes (each one tells the technician when the code was triggered and what the vehicle was doing).
Clears all the codes and performs a test drive under similar conditions obtained from the freeze frame data
Performs a visual inspection if the P0204 code returns
Uses the scan tool to check for proper ECM readings coming from the injector circuit cylinder 3,
Checks the voltage with a multimeter at cylinder 3 fuel injector
If needed, installs a noid light between the fuel injector and its wiring (noid lights measure the voltage pulse width which tells technicians if the injector is opening for long enough)
Performs a test of the ECM if none of the other tests offer anything conclusive.
Read more: https://www.autocodes.com/p0204_jeep.html