2008 BMW M3, 125,000mi, the ODB Readiness monitor for the O2 sensor won't become ready. I replaced the two post-CAT O2 sensors and I'm still seeing the two sensors disagree. One is showing rich and one lean. Occasionally they will sync up while driving, but at idle they always disagree.

The only pending code is for Secondary Air Flow Bank 1.

Next step? I'm thinking about replacing the upstream sensors. Does the Long Term Fuel Trim indicate an issue with the upstream sensors?

Might be hard to read but O21x2 is 0.7V and O22x2 is 0.1V.

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  • Welcome to Motor Vehicle Maintenance & Repair! The maximum the LTFT should be reading is within +/-10%. Once past this, there's a huge problem. This could be a sign of a plugged cat or a leaking injector. You can read more about fuel trims here. Page 3 talks about your specific issue. I'm assuming since your car is an M3, it's turbocharged ... it might be a leak on the cold side piping (post turbo). Jun 16, 2018 at 17:50
  • 2008 M3 is a 4.0L V8 - non-turbo.
    – Jeff
    Jun 16, 2018 at 17:58
  • Well, that's what I get for assuming :o) Anyway you look at it, the computer is seeing it as running very rich and is trying to compensate by taking fuel out using the LTFT. Jun 16, 2018 at 22:05
  • What do the the front sensors read at idle and while cruising? You need a lot more data before you should consider replacing parts.
    – Ben
    Jun 17, 2018 at 4:09

2 Answers 2


A downstream sensor reading lean at idle is often indicative of an exhaust leak. You may find that a flange gasket is leaking or an exhaust flexi has a hole that is allowing oxygen to enter the exhaust system after the front sensor.

A leak like this is most obvious at idle during low exhaust flow, at high flow the problem can disappear, as air is not sucked in through the hole, instead it is blown out.


If memory serves the upstream O2 sensors are the ones that monitor the air fuel mixture. The downstream ones measure the condition of the cat. If they detect O2 the cat is toast or on the way out. One way to test the condition of the cat is using an infrared thermometer and measure the temp at the front end and at the rear end of the cat. If there is a difference of about 100F for a 4 cyl or about twice that for a 8 cyl then the cat is good. Manifold side hotter and after the cat is cooler. Do this after driving for 20 mins or more and leave the engine running while taking the temp.

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