Q: Trying to determine if a Honda 2005 vtec (D172A) sohc, 127 hp has an O2 upstream sensor problem or the ECU

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I've taken voltage reading of both O2 sensors from the ECU connector(s) behind the glove box with digital multi-meter. Seems both initiate at 0.290v bias volts before the Htr circuit effect. Once heated, I observed the downstream sensor avg 0.4 v at idle and acceleration (+/- spikes). The upstream sensor avg 0.295v at idle and accl. With both sensors connected the engine idles fine but struggles on accl, power loss or hunting and when accl released, it gasp and sometime stalls out. When I disconn the upstrm O2 and leave the dwnstrm connected, the engine runs fine on idle and accl. When just upstream O2 connected, the engine runs bad as previously describe. I observed the readings on upstrm O2, it starts out at 0.290v - bias voltage then jumps up to 0.5 - 0.7 v (Htr ckt kicks in?) for a few seconds then drops and avg 0.290v. I notice before the Htr ckts effect (10s) with both sensors in, the engine had full pow at idle and accl. Took upstrm O2 rdg in open air, 0.75v - 0.9v, stable.

My car has 167920 miles.
My car has an automatic transmission.

Hello. From what you describe it would appear that your upstream oxygen sensors are causing the issue. If you disconnect them and the vehicle runs better then it may very well be possible that the signal they are sending or generating may be a bad one, and confusing the computer, causing your performance issues. Keep in mind that the heater circuit and the signal circuit will usually be on separate wires, and should not kick on and off and change the signal of the sensor. Most oxygen sensors are designed to operate in cycles, however, if they are giving off erratic readings that may be a sign that the sensor is bad. I would try to get the specifics on what the sensor voltages should be for that vehicle and compare them with your findings to see what you may conclude. If you need a diagnostic, an expert from YourMechanic can come to your car’s location to inspect the loss of power issue to help guide you through repairs.

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