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OBD1 code 27: sub oxygen sensor

Hi all I'm new here.

2008 Rav4, 4 cylinder, FWD. The CEL, VSC and Track lights are all on. I read JuneBug's article and a few others then did the jumper method.
CEL = 27, Sub oxygen sensor
VSC = 51, Engine control system, which I think is caused by CEL.

My questions,
1. I think there are two O2 sensors in this car, right? Which one is the sub O2 sensor? Is this code enough for diagnosis or do I need to get a OBDII reader?
2. Is it easy enough to replace either or both sensors by myself?

Thanks,
 

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Yes, there are two oxygen sensors, one before the catalytic converter, usually in the exhaust manifold, and one after or in the side of the converter. But they are usually referred to as pre and post or upstream and downstream. I've never heard the term "sub" altho the upstream one is also called an Air/fuel sensor.
 

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On my 09, 4 cylinder, the downstream sensor went bad. Every time I drove on wet roads, the CEL light (including VSC and the anti skid lights) came on and the cruise control would not work. When it dried out these warning lights went away.

The problem was that the downstream O2 sensor had a bad crimp in the housing and water leaked into it, shorting it out. Replacing that sensor was not too difficult and since replacing it, I never have had an issue with it since that time.
 
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