Regeneration will cause the entire engine, the entire exhaust to be hotter. Exhaust temperatures vary wildly on diesel engines because they operate over a wide range of air to fuel ratios which is dependent on the fueling rate which is dependent on engine load, ambient temperature, temperature of the air charge, air charge flow rate, boost pressure, and on and on . . . I don't know why your temperatures are varying but I think it has nothing to do with your DPF. You know you can monitor regeneration status? 0.00 for no regeneration ordered; 1.00 for regeneration requested. You can also look to see what the mileage was at the last five successful regenerations, average miles between successful regenerations, average fuel consumed between successful regenerations, and more. I routinely monitor these values to make sure they're happening regularly, especially since I have both the 452A and 4D4A advisory DPF replacement DTCs.

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8/2011 X5 xDrive35d Sport, Black Sapphire Metallic, Black Nevada Leather, Fine Burr Walnut Trim
2/2001 M5, Jet Black, Exclusive Complete Black Walk Nappa Point Heritage Leather, Black Cubic Trim