I replaced the Oxygen sensor and the error code went away. Was feeling smug about finally not having any "Check Engine" lights, started on a trip and after about 200 miles "Check engine" again. This time it is P0445 Large leak in evap system. Clicked gas cap a couple of times, cleared the code, and it stayed clear for another 200 miles or so, then came back. checked gas cap again, cleared code, got back on the highway, then i get a warning on the message center "gas cap loose" or something to that effect. Clicked the gas cap again, the warning went away but a few miles later the Check Engine is back. Where else can this large leak be?

Your car may have either Bosch or Denso brand O2 sensors. The best bet is to get a look at the original sensor and replace it with the OE brand. Get the proper plug-n-play version rather than the generic version that you have to splice the wires. I like Stockwiseauto.com for O2 sensors as they have the right stuff and generally decent prices.

Determine whether that fault code points to the upstream or downstream sensor. If you know which two wires* feed the Pre-heater then you should be able to measure infinite ohms at the disconnect pins. Also, if it’s a Pre-heater problem it will set a fault code by just turning on the ignition - you don’t even have to start the engine. (Apparently the Pre-heater starts heating when the ignition is turned on. It doesn’t wait for the engine to start.)

* I really miss those wiring diagrams that used to be available on the web.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Thank you Klaus, no idea on total mileage, i have only had this V70 for a couple of years, total mileage around 160K. So if only the heater part is not working, the oxygen sensor is still able to do its job? That is why I mentioned that other than the check engine light, there is no evidence that something is not working properly.

How many miles on the O2 sensor? More than 100K = get a new one. If I remember correctly, the white wires are the heater.
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Keeping it running is better than buying new