('03-'05) persistent o2 heater code P0030 in 2004 Forester - Subaru Forester Owners Forum
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post #1 of 1 (permalink) Old 04-23-2019, 02:15 PM Thread Starter
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persistent o2 heater code P0030 in 2004 Forester

I think I have read ever post online
attempting to diagnose the persistent up-stream Heated O2 Sensor code (P0030)

I replaced the sensor with an Ebay cheap-o
light came back quickly ,
it's heater Resistance was 8.3

so after reading more I went for a Denso (according to the Ebay seller)
light came back quickly
it's heater Resistance was approx. 5.2 (I think this is a good number?)

also
the car battery will drain flat in less than a week,
it is a new battery
It will loose a few volts overnight (batter voltage reading at 11.5 this morning after fully charging last night)

so there is a parasitic drain somewhere

I suspected that the drain and sensor problems may be related
so I started testing more things

the wires from the sensor plug to the ECU both test good
(yellow and a brown with black stripe)

Now what I have noticed today
is that the o2 sensor heater pin has battery voltage
even when the key is turned off !!

so next I went to the ignition relay
(brown color relay under the dash on pass side)

What I noticed is that if I turn the key off,
and test the back of the relay and probe the
yellow o2 heater power wires they remain at battery voltage
regardless if the key is on or off
but if I unplug the relay and put it back the power is now off
and it is also now off at the power pin of the o2 sensor

so I suspect that the relay gets energized when the key is turned
closes contacts and sends the power to the O2 heater
but it remains on so the relay's contacts are not releasing
unless I momentarily remove it from the socket

does this sound like a faulty main relay? or computer issue?
the part number on this relay is 25229aa050


I just replaced all brakes,
rotors and the pass control arm,
but can't pass inspection yet

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