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Old 02-19-2012, 04:43 PM   #1 (permalink)
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Default Check Engine Light came on (code P0156)

I can't get one problem resolved without another one popping up.

Today I noticed the check engine light come on. I plugged in my code reader and it's showing P0156. The book says it's the 02 SENSOR CIRCUIT MALFUNCTION. I drove around most of the day with the light on. Then about a half hour ago I was out driving again and the light went off.

So I'm wondering whether I should be concerned about this or not. Could this be an early sign of something bad about to happen?. I guess it's a good thing if the light stays off.... but there had to be a reason the light came on in the first place.

Car is 2003 Taurus with 3.0 Vulcan. 120,000 kms (74,000 Miles)

PS. I should point out that I still have the front timing cover leak. Just topped up the coolant today. Not sure if this would have anything to do with this 02 sensor

Last edited by NorthernTaurus; 02-19-2012 at 04:49 PM.
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Old 02-19-2012, 05:09 PM   #2 (permalink)
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The code could mean the O2 sensor is on it's way out. If you have coolant or oil leaking on the O2 sensor it can cause them to go bad. Bank 2 sensor 2 is the post-cat sensor on the radiator side of the engine.

Last edited by behlinla; 02-20-2012 at 01:04 AM. Reason: Wrong side!
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Old 02-19-2012, 10:31 PM   #3 (permalink)
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Bank#2 is radiator bank. Sensor#2 is indeed post-cat sensor on that bank. Check elec. conn. and wiring for damage.

Last edited by sheila; 02-21-2012 at 12:23 PM.
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Old 02-20-2012, 10:07 PM   #4 (permalink)
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Thanks for the replies.... but I'm confused.

The only code I got was P0156. How can you tell whether it's the "Bank#2 radiator bank" or "Sensor#2 post-cat sensor" ?

Are either difficult to replace?. Any diagrams?.

The light hasn't come back on since yesterday. What purpose do these sensors serve?
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Old 02-20-2012, 10:14 PM   #5 (permalink)
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The second 'downstream' sensor's function is only to make sure the cat is functioning properly. If you are only getting that one O2 sensor fault code I would inspect the wiring, making sure it's ok, and if all is good replace the o2 sensor.

I had 4 O2 heater circuit malfunction codes on my Tbird and it ended up being a bad PCM. This doesnt sound like the case if it's only the one.
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Old 02-21-2012, 03:39 AM   #6 (permalink)
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I can tell what sensor it is because that's the definition of P0156 = Oxygen Sensor Circuit Bank 2 Sensor 2. For example. P0155 is Bank 2 Senor 1, etc. The bank is which side of the engine the sensor serves (bank 2 are the three cylinders on the radiator side), and the sensor number tells us if it's before or after the catalytic convertor. Sensor 1 is upstream, and sensor 2 is downstream. Your car has four oxygen sensors.

Like the name implies, oxygen sensors sense the amount of oxygen in the exhaust stream so the engine computer can determine the current mixture ratio of the engine (whether it's rich or lean). The downstream sensors are there just to make sure the catalytic convertor is working by testing the convertor's oxygen storage capacity. If you want to know more, just Google.

You have a circuit code, which might mean the wiring is damaged, disconnected, or melted. Just follow the exhaust and find the sensor below the first catalytic convertor on the front side. They screw into the exhaust pipe and there is wiring coming out of the end of it.
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