P15b3 p1446 fault codes

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Hi, so I have these two codes come up on my Peugeot boxer, 61000 miles 2016, additive adding is all I can find, using an icarsoft pro so I’m not sure exactly what it means, any ideas on how to fix it or what it actually means, I’m presuming after reading lots it’s def fluid? But not 100% sure, thanks in advance for any help. Chris.