'89 420SEL - "check engine now" light

my 89 420sel has just lit it's 'check engine' light - I'm led to believe it more than likely means the o2 sensor needs replacing. Can anyone give me an idea of how likely this ? How could I confirm that
it is / is not the o2 sensor ? Any ideas, anyone ?
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The boyz in tha Bosch o2 sensor works state that the current crop of sensors are good for 150,000 miles. Maybe you should just replace the old ones and drive it some more.
mcbrue boschily under the bridge in the trailer down by the river
96 S420
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