For your info, key on 70% indicates that yours is a federal car.85% would indicate it is a Calif car.These are indicators of the emission standards adopted for a particular car.
Engine cold 50% indicates that car is still in open loop ie O2 sensor not warmed up.
A duty cycle of 18% fluctuating
at engine operating temps would indicate a very rich mixture.
Larry is right on the money.
duty cycle at operating temps indicates a fault code.
18% is assumed as 20% (as the fault codes are in increments of 10%).
The only code I know is a fixed 50% code at operating temps indicates a faulty O2 sensor.