2006 EXL 4dr, 4cyl with 162000km.
CEL came on, Honda shop diagnosed it as P2271 & P0139.
- ran a tank of premium with fuel cleaner, cleared the code, but it returned.
- changed the downstream O2 sensor with one from NGK, cleared the code, but it returned.
- changed the sparkplugs with new Iridiums, cleared the code, but it returned.
- changed the downstream O2 sensor with a factory one, cleared the code, but it returned.

I'm running out of the cheaper fix options, does anyone have any other ideas?

 

Code P0139 is secondary O2 sensor slow response and P2271 is secondary O2 sensor stuck rich (high voltage, 0.7-0.9V).

Since you've replaced O2 sensor twice, the most likely explanation is a wiring or connector fault causing constant high voltage on signal lead. Also possible the fault could be an internal PCM fault.

To diagnose you will need to disconnect PCM from harness w/ secondary O2 sensor wiring (shop manual req'd to ID connector and wires) and disconnect secondary O2 sensor from wiring harness. Check integrity (free of shorts to voltage or ground) of O2 sensor signal and reference voltage lines from PCM connector to O2 sensor connector.

If all wiring checks good, then substituting a good PCM is final step.

A shop manual is essential for DIY (automanualsource.com for $22).

good luck

 

In addition to the above sometimes the MAF is the culprit. Suggest cleaning the MAF. Sometimes the code for the MAF will not show up, but due to its relationship with the air/fuel ratio it can be the cause for other codes within the system.

 

Thought I would respond back to my original thread.
The Honda shop service rep had suggested I just run with the code active as replacing the ECU was the next logical (and expensive) step, one that was still not guaranteed to correct the fault.
In the long run, we finally did find a solution to the problem by...
just fuelling up at a different station, after that, the code even cleared itself.


To further test things, we have gone back to the offending Esso station twice in the past four months, and each time, the engine code showed, but again cleared itself once the offending fuel was replenished at a different Esso or different fuel company.


Sometimes the obvious is difficult to see due to its simplicity.