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Guest Message by DevFuse

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2000 outback CEL p0420 & p1139 (o2 sensor)

catalytic converter

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3 replies to this topic

#1 aanthon23



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Posted 02 November 2013 - 05:53 PM

Hi, I am new to this forum. I was hoping to get some advice on my new outback. Its a limited 5spd with 141k miles. the check engine light has come on and gone off a few times since i bought it, but now its been on for a while. i borrowed a code reader and it came up with the all too common p0420 code meaning the catalytic converter isn't operating at effienciency. It also showed code p1139 or "oxygen sensor heater curcuit malfunction"


I was hoping this means that if i replace the o2 sensor the p0420 might go away too. I'm looking (and praying) for a cheap fix, i really dont wan to have to replace the cat.


Has anyone else dealt with this issue or a similar issue? should i replace the o2 sensors or might these two codes indicate a deeper problem?


any advice is welcome, thanks

#2 davidsain



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Posted 02 November 2013 - 07:28 PM

The HEGO sensor has a heater in it that it needs to function.  When the heater circuit fails, the sensor can no longer burn off deposits. I've replaced a few on my 2k wagon - they fail through normal catalyst depletion.  Replace this first, clear your codes and see what happens.  As the sensor has a buildup of deposit, it won't read correctly.  There is a rear sensor to determine catalytic converter performance and the failed front could cause this.


If you do end up replacing the cat, realize that a bargain cat isn't a bargain.  A bargain basement cat will have less precious metals that make it what it is and can have a significantly shorter lifespan than OEM.  Replacing cheap cats every 50K miles is more expensive than once every 150K.

#3 aanthon23



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Posted 03 November 2013 - 11:42 AM

thanks davidsain, I will try this asap and let you know what happens. and thanks for the advice on catalytic converters too

#4 Fairtax4me



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Posted 03 November 2013 - 01:41 PM

P1139 : Front oxygen sensor heater circuit problem
A problem with the heater circuit or wiring to the circuit may affect overall performance of the sensor. Its likely that if the heating element in the sensor burned out it damaged the sensor element in the process, which could be causing the P0420 code. You should check the wiring and connector to the sensor before replacing. Also make sure the sensor housing is not covered in grease from a split CV boot, as that can cause problems with the way the sensor reads. If the boot is split replace the boot (or the axle) before replacing the sensor. If grease from the torn bolt is slung on the new sensor it may damage the new sensor as well.

The heater is not designed to burn off buildup from the sensor element. It merely warms the sensor to operating temp (about 650 degrees) faster than allowing the exhaust to warm the sensor on its own. Since the O2 sensor doesn't work until it reaches operating temp the ECU has no feedback on exhaust oxygen content until the sensor is warm, so it cannot fine-tune the air/fuel mixture to compensate for an overly rich or lean condition. To reduce possible emissions the heater is there to make the sensor warm up and start working faster so the ECU can fine tune the A/F ratio sooner. Often once the sensor reaches operating temp the heater is turned off.

Also tagged with one or more of these keywords: catalytic converter

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