Did you mention a vacuum leak in the brake booster? It's called a vacuum
leak, but in fact, it's an air leak, admitting 'unmetered' air into the
manifold. If that's what you have, no wonder that your oxygen sensor reads
In general, the fact that a sensor reads the wrong value, does not
necessarily mean that the sensor is bad. It could simply be reporting the
truth. Consider this: you mesure a low voltage across the battery terminals.
Do you replace the voltmeter, or try to figure out why the battery is