Intermittent engine light.

56 reg TD4 with 75K on the clock. Just lately we have started to get an intermittent engine light. Mostly when driving up hills on the motorway? It goes off when the engine is no longer under extra load? i have tried reading the codes and non are stored. Im presuming it is emissions related? If it is any ideas? TIA. Car is running fine BTW?

All you need to know is in this stucky thread.
https://www.landyzone.co.uk/land-rover/td4-lack-of-power-at-70mph-fixed.77147/

Many thanks i will wade through that.