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Please refer to Catalyst Temperature PIDs (mode 01 pids 3C, 3D, 3E, 3F) for the following questions - https://en.wikipedia.org/wiki/OBD-II_PIDs#Mode_01

Q1. Is there a thumb rule that the catalyst temperature at bank 1 will always be greater than or equal to bank 2 catalyst temperature?

Q2. What is the catalyst temperature range that can be considered normal for petrol and diesel cars? I googled it, but different websites had different ranges.

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    If the engine is running correctly (ie: all cylinders firing equally), logic would dictate cats for both bank 1 and 2 should be approximately the same temperature, everything else being equal (ie: cat efficiency, etc). As far as cat temps for vehicles, this is going to depend on a lot of factors, such as type of catalyst used, grade of fuel, efficiency of the engine, forced induction or naturally aspirated, manufacturer, and fuel type (list not all inclusive). This is probably why you aren't finding a solid answer. I don't have a referenced answer for you, so leaving this as a comment. – Pᴀᴜʟsᴛᴇʀ2 Feb 10 '17 at 12:34
  • As per @Paulster2, but also consider the area that each cat is located - without any car/model detail, then one cat could be in a "colder" place or in more draught than the other causing a temp difference. The reference only shows the min/max possible values : do you have any real data that supports your question or an problem with error codes that can be seen? – Solar Mike Feb 10 '17 at 22:22

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