2001 Legacy reads Po420 code and front O2 sensor reads 3-4 volts

[quinnrj]

I am driving a 2001 Subaru Legacy Wagon (stick) with 137,000 miles.

 

The CEL is on, and I'm getting the P0420 code - Cat efficiency (bank 1).

 

I checked for exhaust leaks (I used to have one behind the cat, but sealed it) & replaced the rear O2 sensor with an OEM Denso bought from Amazon a few weeks ago. All of the pipes seem to be fine (tapped them with a hammer).

 

I brought it into a shop today to do a diagnostic, and their equipment shows that the front O2 sensor reads 3-4 volts (BTW the same sensor was replaced in July 08 with an OEM Denso bought from RockAuto.com). :-\ The shop didn't charge me for the Diagnostic (thanks, PepBoys!), and told me that I should bring it to a Dealer to do a more detailed diagnostic.

 

The tech that did the Diagnostic also didn't see any leaks. He suggested it could be a computer issue, or a wiring issue (melted or damaged). The car was in an accident in Sept. 04 - long before this issue came up. He suggested that the heat shields on the Cat were loose, and that Subaru will probably recommend replacing the Cat (for that reason).

 

I'm planning on dropping it off at the Dealer's tomorrow night for their $98 diagnostic, but I thought I'd post here to see if anybody has any suggestions I could try in the meantime. I'd read on some forums that re-flashing the ECM by the dealer can sometimes fix the Po420 code.

 

FYI: The car was winterized in July '07 & stored for a year (we moved to England for a year). After pulling it out of storage in Sept. '09, I had to replace the clutch, front brakes & the Idle Control Valve.

 

Any thoughts would be appreciated!

 

As an aside - I've seen software & cables advertised online for hooking one's laptop up to one's car & reading Diagnostic info. I'm enough of a geek to think that's cool. If anybody has suggestions for a cheap cable/software that I could buy, I'd be interested in that also.

 

:-\

Edited October 24, 2009 by quinnrj

[Fairtax4me]

3 full volts?

 

It should sweep somewhere between 0.0 and 1.0V. Maybe they said 0.3 - 0.4V?

[quinnrj]

3 full volts?

 

It should sweep somewhere between 0.0 and 1.0V. Maybe they said 0.3 - 0.4V?

 

He showed me on his Snap-on diagnostic machine it was between 3.5 and 4.1 Volts. The rear O2 sensor was reading between 0.0 and 1.0 V.

 

Thanks for the quick response.

[OB99W]

You might find http://www.ultimatesubaru.org/forum/showthread.php?t=66744 interesting.

[quinnrj]

You might find http://www.ultimatesubaru.org/forum/showthread.php?t=66744 interesting.

 

Thanks for the suggestion. I'll take a look for a loose wire, and if I have time, pick up a torch to check the sensor with the Torch test.

 

As in that post, the sensor's voltage did increase when throttled.

 

-Bob

Edited October 24, 2009 by quinnrj

[OB99W]

I suggested that thread because it showed that the voltage reading didn't indicate a fault. Some generic OBD-II scanners don't read certain A/F sensors correctly, or the data is misinterpreted. (By the way, that's true in particular for wideband type.) Pep Boys might have suggested the dealer because they would use the Subaru Select Monitor instead of a generic scan tool.

 

P0420 can have several causes (besides the cat itself being bad) -- do a search here for that code, and it should turn up threads with more info. Here's a link to some: http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120 .

[Log1call]

Yup, that code will almost certainly be caused by something other than an electrical problem. Well an O2 sensor electrical problem anyway. If the O2 sensors were faulty you would get a code for the sensor itelf. That code is saying that the cat isn't cleaning the exhaust properly, a conclusion arrived at by monitoring both sensors and looking for a difference in their two readings.

 

As long as both sensors swing through the centre of their range, and the rear one swings through a comparatively smaller range(it may be a five volt sensor) and at a slower rate, then the code wouldn't be set and the sensors would be operating correctly.

 

Some cars use a five volt sensor but I thought those were normally on the back sensor, I may be wrong about that or they may have been reading the back as the front or vis versa.

 

The way to test the sensors is to warm the motor, leave the sensors connected and monitor their voltage swings. The front one should swing through the centre point of it's voltage range several times a minute and the rear one should swing through a smaller range and slower.

 

If the rear sensor isn't swinging slower than the front then the cat isn't working.

 

Have a search through here... http://endwrench.com/main.php?smPID=PHP::search_articles.php

 

Look in the "select a system", "fuel and emmissions"

Edited October 24, 2009 by Log1call