Using 'When' + past participle 2

Is this sentence (a caption for a question/article) grammatically valid?

'Getting 403 http code when routed to the site's root'

When is it suitable to use WHEN + PARTICIPLE Ⅱ? Should I use being routed instead?

That aspect of your 'sentence' is correct. (It isn't a valid sentence because it doesn't contain a verb.) But certainly 'when + past participle' is good everyday grammar. e.g 'When told she was overjoyed', or 'When excited he is liable to do something silly', or 'I shall be happier when informed of the results'.

There is no Participle II in English; the past participle is the Participle III. Remember that for regular (weak) verbs in English, the second and third principal parts are identical, so you need only pay special attention to the use of irregular (strong) verbs’ participles.

WS2’s examples all are good, but all are examples of regular verbs. Here are some examples using irregular verbs:

• “When blown off course, control is impossible.”
• “Vegetables crunch when eaten raw.”
• “While being shown to his table, he tripped.”

As for when to use being, that issue is really too complicated to cover comprehensively, so here’s the short version.

If you want to use the word being, use while or whilst instead of when. While being routed would mean that the 403 happened at the same time as the routing, not as a result of it.

Use when routed to convey that the routing happened first and the 403 code came as a result.