How can I confirm the failure/cause of the O2 Sensor Failure? It's getting awful tempting to conclude head gasket induced anti-freeze leak and go down that rabbit hole.

I have a 1990 California Model 300E, with the 103 3.0L engine. The engine has 137,000 miles. The Oxygen sensor was replaced last year, at 122,000 miles with a new Bosch OE sensor.

Observations:
1. The car runs smooth at an idle of 500 RPMs in gear. I've received Code 18, idle air control valve, at intervals decreasing from once in 60 days to weekly over the past year, so I ordered a new Idle Air Control Valve that should arrive soon. I hope to see the idle raise to something more than 500 RPM, and the code 18 go away.

Mileage is running at about 17mpg/gal in mixed city and hiway driving, using premium.

Antifreeze is dissapering at a rate of one inch out of the reservouir in a 30 day period. There is no observed steam coming from the exhaust. There is no observed leak from the water pump, sensors or hoses.

Temp is running slighly over 80 C, raising in prolonged traffic to about 110C when the Aux fans kick in and bring it back down to 80C.

The timing chain cover is starting to show signs of oil weep.

I have not used any silicone sealants on that vehicle.

No oil consumption - Oil level remains full during the whole 3,000 mile/3 month change interval I follow.

You may want to change the thermostat and cap and check for leaks around the expansion tank, hoses, radiator, water pump...

You will know when you need a headgasket with the mucky muck in the expansion tank/cooling system - oil in the coolant.

Maybe just the cap letting hot coolant flow from expansion tank to overflow tank in fender.

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