O2 Voltage

The O2 sensor liklely has a heater built-in so it can get up to operating temp (>650F) in a hurry. The heaters often burn out on old O2 sensors, but the sensor still can work if the exhaust can make it hot enough. That's usually no problem when the engine is operating above idle speed.

At idle the exhaust temps are cooler so, depending how far the O2 sensor is from the heat source, the O2 sensor may cool off enough at idle to not work properly.

If the O2 sensor voltage comes up to > 0.7V when you rev the engine a little then its heater is probably bad and the sensor will have to be replaced (unless you don't mind the check engine light on).

I tuned a 2G one spring day and its O2 stopped working each time I let the engine idle! I checked and its heater was bruned out.

Rick