P2A00 on a new O2 sensor – 2005 Honda Civic LX

[rt54321]

About 2 months ago, the car had a check engine light on. I disconnected the O2 sensor from the harness and measured heater element resistance – it was open-circuited. The new O2 sensor had a resistance of something like 1.6 ohms (I’m recalling this from memory), so I am confident that the old O2 sensor was bad.

Replacing the O2 sensor turned the check engine light off, and everything was working fine for about 2 months. Now, the check engine light is back on with a P2A00 fault (“O2 sensor output is out of range”). Did I buy a “bum” O2 sensor? Are there things I can test (O2 sensor output, heater resistance/heater current, O2 sensor is loose, etc.), before I replace the O2 sensor again? I just find it hard to believe that the sensor was working for 2 months before causing another fault code – but I certainly could be wrong.

Thank you for your help!
-R

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[Richard Kirshy]

A couple of thoughts here..

First, did you install a OEM sensor?>
Honda can be a bit finicky about replacement parts.

Next ,I believe there is a TSB concerning a re-flash of the ECM to help with this issue.

Lastly, you may want to check the connector on the sensor.
Clean both halves and apply some die- electric grease to both halves.

[rt54321]

Thank you for your reply! Quick measurement updates:

1. The O2 sensor is NOT a Denso (I understand that Denso was the OEM for the upstream O2), so indeed – perhaps I should have gotten a Denso O2 sensor.

2. I didn’t find the TSB for my specific Honda car, but I don’t have access to all resources either. I can only lookup recall info.

I took some measurements with the car running – the voltage reference wire was measured at a solid 2.08V, and the signal wire was measuring between 1.65 and 1.9V (rapidly changing). These measurements were taken with a voltmeter. I understand that an oscilloscope waveform would be preferable for measuring the O2 activity, but at least the 1.65 – 1.9V signal shows me that the O2 sensor is “alive”.

[Author]

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