C4 Picasso B78 2.0 Blue Hdi Access to exhaust temperature sender

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Does anyone have the official method of replaceing the exhaust gas temperature sender that is fitted just before the turbo on the Blue Hdi 2.0 ? Can't see it; can just about feel the connecter over the top of the engine but all in all looks a serious pig of a job. Any tips welcome. I am not entirely sure if it does need changing ---- there is no fault code coming up, but a real time read out when driving shows just a flat line with no change regardless of whether the engine was under load, coasting etc.

As I have a new one for comparison, does anyone have the relevant terminal numbers at the ECU plug in order I may test the resistance of the existing one ?

Thanks, Mike.