The primary lambda sensor is dead or the plug fell off.
In most cases, you need to plainly replace it with the exact type required for that ECU.
Generic lambda sensors won't work as a primary lambda sensor on the Honda ECU.
Following troubleshooting for the '02-'03 RSX:
-Check the A/F sensor (the one before the cat), is it loose?
-Reset the ECM, clear codes
-Let the engine run until the radiator fans come on (or go for a drive to warm it up)
-If the fault comes back, replace the sensor.
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