If I buy a set of 4 upstream and downstream 02 sensors how do I know which ones are upstream which are downstream

SOURCE: Installing 02 sensors

"Up-stream" means before the catalytic converters and are probably in the header pipe(s). The "down-stream" O2 sensors are behind the cats in the exhaust system, usually just behind them. Not the expert here on this particular car but there are probably 4 of these sensors. BMW exhaust systems are very tight on space, be careful tackling this job.

Mike Butler

• 6674 Answers
• Posted on Jan 11, 2009

SOURCE: 1997 Dodge Stratus V6 oxygen sensor location

Upstream, as in ahead of the converter, back toward engine. Start at converter and work your way back, should be on exhaust pipe or even on manifold.

SOURCE: oxygen sensor confusion

1/1 is upstream sensor on the number 1 cylinder side, 1/2 is the downstream of the number 1 cylinder side, 2/1 is the upstream of the number 2 cylinder side, 2/2 is the downstream of the number 2 cylinder side. Hope this helps.

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SOURCE: Getting PO141 trouble code when scanning! Car '03 accord, 2.4 ltr

i spoke to a honda tech , and he confirmed code p0141 on the 4cylinder model is the second sensor.

SOURCE: 420 code catalyst system efficiency below

You could have an enefficient converter. Question is what caused it to go bad? I have attended seminars on code 420, Possible causes are lack of spark or clogged fuel injectors. there have been times where the spark plug wire lays on the engine and over time it wears a weak spot in the side of the wire, its not enough to throw a miss fire code but it keeps the full amount of spark from getting to the plug and not burning the all the fuel in the cyl. A leaky injector would also cause to much fuel to be deliverd into the cyl and not get burned.

Hope this helps.