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Bad O2 Sensor


cgholson
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I have a 91 LS 400 and my check engine light is on. I took it to Auto Zone and the code came back 28 bad O2 sensor before the converter on the right. The way I understand the exhaust system there is a sensor in front of the converter and one behind. Auto Zone said I needed to get 2. Are they saying I need to replace the front and back because both of them are bad or do I just need to replace the front one?

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The front sensor is a coarse adjustment and the rear sensor is a fine adjustment. They may be suggesting doing both because they are probably both showing signs of wear. If doing it myself, I'd just replace the bad one and then watch the performance. On our Volvo the A2D interface for the sensor inside the computer that reads the second sensor is dead (new computer is > $1500!!!) and it will run for about a week in a backup (but still closed loop) mode. When it gets tired of having no second sensor it trips the system out of closed loop (like having no ox sensors at all) and sets the "check engine" light. Considering the cost of repair, I simply reset the system to clear the "check engine" light and put it back in closed loop mode once a week. Note: Ox sensors come as generic (just wire pigtail that you have to splice) and OEM with a custom harness, cost is 2X to 3X higher for the custom harness. OxygenSensors.com shows front (pre-cat) OEM Denso for $92, OEM harness Walker brand for $80 and generic for $50. I don't care for the connection kit they supply for the generic sensors and I solder and pot the connections, but be sure to break the harness connection at the OEM harness connection if you do something like that or you might fry your computer. That isn't what happened to the Volvo, but I'd bet that a new computer for the Lexus is in the same price range.

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I changed my main O2 sensor witht hat code. www.lexls.com shows the resistance that should be reading on the O2 sensors in their O2 sensor tutorial. I got a Denso O2 sensor, same as the one they came with, but no plug on it off of Ebay for $30. I also got a universal one at the same time, and sent the universal back as it's resistance reading was way different than the denso, which was close to bang-on.

In Canada, I got quote $410 for the O2 sensor part, no install from NAPA, and at Canadian tire the same deal is $295 but for a universal, so I have to recommend Ebay for them.

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