Hi folks,
I’ve been struggling with poor fuel economy on my 2007 Honda Civic EX for a while now (only 500 km or less per tank in the city). I suspect the issue is related to the upstream O2 sensor, as I’ve been checking its voltage and behavior.

Here’s what I’ve observed:
• Upstream O2 sensor (S1): No voltage between the two heater pins, whether the ignition is on or off.
• The hot pin is always connected to 12V, but the ground pin isn’t connected to anything at any time.
• Downstream O2 sensor (S2): Voltage is present between the heater pins when the ignition is on.
• The hot pin is always 12V, and the ground pin connects properly when the ignition is on.

Other details:
• There’s no check engine light, and an OBD scan showed no error codes.
• The upstream O2 sensor was replaced, but everything remains the same.

Questions:

1. How can I fix the heater circuit issue on the upstream O2 sensor?
2. Could this be related to a blown fuse, poor wiring, or something else?
3. Any other tips to diagnose or improve the fuel economy on my Civic?

Thanks in advance for your help!