A P0204 code on your 2019 Nissan Altima indicates the ECM has detected an electrical problem with the cylinder 4 fuel injector circuit. Since the Altima's four-cylinder engine has only four injectors, cylinder 4 is the last in the firing sequence.
Cylinder 4 Location on the Altima
The 2019 Altima with the 2.5L four-cylinder QR25DE engine has cylinder 4 located at the transmission end of the engine block—the rear cylinder when looking at the engine from the front of the car. The injector connector faces toward the firewall side of the engine.
What Triggers P0204
The ECM monitors injector circuit current and timing. P0204 sets when the cylinder 4 circuit shows an open, short to ground, short to power, or unusual current draw. The injector coil failing open is common, as is connector corrosion from moisture exposure in the rear engine area.
Diagnostic Process
Check the injector connector for corrosion—the rear cylinder location makes it susceptible to splash from the road. Test injector resistance (should be approximately 11.5-13.5 ohms at room temperature for this engine). Verify the ECM is commanding the injector using a noid light or oscilloscope. Check wiring for damage between the connector and ECM.
Repair Approach
If the injector has failed, replacement requires removing the fuel rail. Depressurize the fuel system first and use new O-rings. If corrosion is the issue, clean terminals and apply dielectric grease to prevent recurrence. Verify repair by clearing codes and monitoring for return over several drive cycles.