LloydMd:

Just as a refresher first, the duty cycle (aka on-off ratio) of the engine control unit (ECU) is a means of checking the response of the mixture feedback loop to fuel/air ratios as sensed by the oxygen (O2) sensor.

When the O2 sensor is cold immediately after engine start, the system is not receiving valid information from the O2 sensor, and it remains in the open loop mode. In this mode a fixed cycle/on-off ratio of approximately 60% will be observed.

When the engine and O2 sensor have warmed sufficiently (2 min. or less), a valid output from the O2 sensor will cause the system to switch to closed loop mode. When the loop is closed the ECU operates with feedback information and continuously varies the fuel/air ratio to bracket the stochiometric ratio (Lambda = 1).

When the system is operating in closed loop the duty cycle/on-off ratio will be observed to swing between 45% and 55%. If the ratio is not cycling there is a fault in the system. The very high ratio of 98% corresponds to either a failed O2 sensor, or a wiring fault that is interrupting the O2 signal.