2003 Baja 5 speed N/A with the dreaded P0420

[NuclearBacon]

Hello USMB!!!

 

I have a 2003 Baja 5 speed N/A with the 2.5L engine.  I just put in a new (rebuilt) engine and new (rebuilt) transmission (150 miles now on the running gear).  The reason for the new running gear is a story in itself.  It now runs great with a nice amount of power and a good idle.  The problem is the dreaded P0420.   I have replaced the rear O2 sensor with a new Denso (the correct one), however the P0420 still returns.  It easily sets all the monitors (53 MPH for 3-5 minutes with 1/4 to 3/4 fuel and no speed variation or brake tapping), but at the same time throws a "pending P0420" thus I know the "check engine light" is not far behind (and, of course, it was).

The readings are as follows.   The front A/F sensor consistently reads 2.0 to 2.1 volts with an occasional dip to 1.9 volts.  A very smooth sine wave.  The fuel system remains "closed" during these runs.   The rear O2 sensor (unfortunately) reads from .1 through .9 volts.  The graph of the rear O2 sensor looks like the teeth of a saw between .1 and .9 volts instead of a nice gradual sine wave between .4 and .7 or .8 volts.  The new and old O2 sensors read about the same.  Backing off down a small hill under compression with my foot off the throttle produces a front A/F sensor voltage of 5.1 volts, a rear O2 voltage of .000 to .005 volts, and an "open" the fuel system.  Any throttle pressure on that hill almost instantly returns the readings to the ones already listed.

The exhaust temperature going into the catalytic converter reads around 390 to 420 degrees.  The exhaust temperature leaving the catalytic converter reads about 490 to 510 degrees.  That leads me to believe the catalytic converter is probably working OK.   My only hope (short of buying a new catalytic converter - $$$$) is the front A/F sensor is faulty and reading incorrectly.   Is a consistent voltage reading for the front A/F sensor of 2.0 to 2.1 volts the correct range for a 2003 Baja 2.5 non-turbo.  I've read that the range should be 2.4 to 4.0 volts, meaning I may be running too rich (see LTFT and STFT below), possibly due to a bad A/F sensor.   I failed to mention I have already checked for vacuum leaks, exhaust leaks, etc.  All is well.  I also have fresh premium Chevron fuel in the tank.

Again, the engine runs and idles as if nothing is wrong.  Instantly starts, nice amount of power (still new so I haven't pushed it super hard), hits 70+ on the freeway with light throttle pressure, good acceleration from a stop, etc.  Nothing should be wrong or is wrong with the engine.   I'm leaning toward a bad front A/F sensor giving me a reading outside the computer's parameters, thus ultimately causing my P0420.  I haven't dealt with the fuel filter, injectors, fuel pressure regulator, etc because there are no driveability problems, thus I feel it is reasonably safe to ASSUME (I know) the fuel system is OK.   My LTFT and STFT all are in the -2 to +2 range, mostly hovering around zero.  I would think a fuel system problem ultimately causing my P0420 would show up in the fuel trim readings.

What is the correct voltage range for a front A/F sensor on a 2003 Baja 2.5L N/A?  If mine is out of range, do you think a new A/F sensor ($130) might be my solution?  If not, what do you think might make my P0420 go away so I can get my smog check and my current tags?  I don't want to use a spark plug anti-fouler on the rear O2 sensor because the smog stations check for that here in California, thus I would fail my smog check the minute they saw it (as you know, it is easily seen from the engine compartment).  

Could someone with a Baja, or Legacy with the same engine and no check engine light illuminated, go for a quick ride with their scanner hooked up to read the front A/F sensor, then report back to me their voltage figures?  That real world test would, at least, give me a starting point to solve this problem instead of just throwing parts at the car (read rear O2 sensor).  

 

Thank you!

[brus brother]

http://www.subaruoutback.org/forums/66-problems-maintenance/49537-p0420-diagnosis.html

Have a good read at the above link.

Seems like you already have some sort of scanning ability but if not, for a $9 sensor and free software like this one LINK, and your laptop computer, the Outback Forum will help you evaluate your readings.

Cardoc is especially responsive and helpful.

BTW, can't you clear the codes and get the systems "ready" then drive 40 mph to the test center before the pending code returns?

Edited September 10, 2013 by brus brother

[Fairtax4me]

Front O2 sensor should have a quickly changing voltage. Should also be between 0.1 and 1.0 volt.

Or in the case of some sensors it should be closer to the 3 volt range.

Are you sure you checked the sensor output and not the heater circuit?

 

Little or no change in voltage means the sensor is bad or the fuel system is not cycling between lean/rich as it should. Usually its a bad sensor. Since the rear sensor is reading a quick voltage change, I think its safe to say the front sensor is bad.

 

Have to have a lean and rich cycling for the cat to work properly.

[GatorIsaac]

I'm pinging this thread because my 2003 Baja is also throwing the P0420 code. It frequently resets itself after a few hundred miles. Easy for me, I commute at least 150/day. My scanner shows the pre-Cat sensor voltage between 2.1 and 2.9 V. I cannot verify the reading for the instant the code is thrown. The voltage mentioned above was measured when no code was active.

2003 Baja

No turbo

5 speed (from 2007 WRX)

230,000+ miles

[Fairtax4me]

Does your scanner show freeze frame data? That would show the conditions present at the time the cose is set.

 

2.9v is low. With that style of sensor that indicates a rich mixture. Or the sensor is failing and is giving a false rich reading.

Check the post cat sensor (S2) voltage. If the post car sensor voltage indicates a rich mixure you may have a fuel system problem. (Clogged injectors, or bad MAF/MAP sensor signal)

If it indicates a lean mixture the pre-cat sensor (S1) is bad.