Congrats on finally fixing your problem. Bosch is the Volvo OE sensor and known as good quality. It's always possible you got a faulty one. Switching from a Bosch O2 sensor to an NGK sensor is like switching spark plug brands and really shouldn't have made such a notable difference.

There's something different about your re-installing another sensor. I'm rather curious what it might have been.

o Did you clear the Check Engine light after installing the Bosch sensor? Did it come back and how soon did it come back? Right away or a few moments later?
o Did you or your mechanic try taking a voltmeter meter to the Bosch O2 sensor to see if the voltage was within spec? Did the O2 sensor voltage response get noted when the engine was run up to higher rpm? If the Bosch sensor was faulty it likely would have failed these simple checks.
o Which Bosch O2 sensor p/n did you install? New or used? Was it a generic one with pigtails or did it have the full connector cable? Did you happen to incorrectly apply dielectric grease or electrical paste to the connectors?
o Did anything else get replaced or done at the same time that you removed the Bosch sensor and installed the NGK sensor? Like find a cracked/loose vacuum hose? Disconnect/clean/reconnect any of the other electrical connectors again, especially the AMM? Clean/adjust the throttle body again or idle control valve? Pull the ignition wires and reseat them? Remove the spark plugs?

Just grasping at a few straws there in case it triggers a thought with you.


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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now