My 2018 E Class Cabriolet (46,000 miles) is equipped with a 10-liter AdBlue tank, and I have consistently replenished it with 5 litres of AdBlue, including an anti-crystal component, immediately upon the activation of the warning sign.
This week, it determined that it could no longer interpret the level. P13E7FD error code reappears immediately upon clearance.
I lowered the tank, discharged 6 litres of AdBlue, then inserted a borescope to inspect for any apparent issues. I saw crimson specks scattered across the area and on the white mesh at the tank's base, however there was no apparent issue. I anticipated crystals or a similar substance. I purged it with AdBlue, replenished it with 4 litres of fresh AdBlue, and reconnected it.
No satisfaction.
Frustratingly, once clearing the code, the dashboard display correctly indicates 4L of fluid for a brief while before reverting to 'Cannot measure fluid level.'
A new tank, with a pump that is unnecessary due to the absence of any faults, costs £1300.
A comparable used tank of similar age is priced at £250 on eBay; hence, I will pursue that option first.
I wanted to share this in case anyone believe they can successfully 'clean' the tank to resolve a level sensor malfunction.