Passive form of a sentence that has "promise to [verb]"

Question

Sentence:

The manufacturer promises to repair or replace the product if necessary within warranty period

I want to convert the above sentence into passive. Is either of the following sentences the correct passive form?

1. The product was promised by the manufacturer to repair or replace if necessary within warranty period
2. The product was promised to be repaired or replaced if necessary within warranty period by the manufacturer

I feel like sentence 2 is the right one, but that is my feeling as I am not sure.

For sentence 1, I only changed "promise" to passive form while for sentence 2, I changed all of "promise," "repair" and "replace" into passive form.

So, can you tell me what is the right way to change the above sentence to passive form?

Answer 1

The manufacturer promises to repair or replace the product if necessary within warranty period.

Here, the manufacturer is the subject of promises, repair and replace.

If you only change one verb to the passive like this:

The product was promised by the manufacturer to repair or replace if necessary within warranty period.

You have introduced a problem in your sentence. The subject of the sentence is now the product, and repair and replace now also have the product as their subject. But the product will not repair or replace itself!

You have to change all the verbs to correspond with the subject, as you did in your second sentence:

The product was promised to be repaired or replaced if necessary within warranty period by the manufacturer.

You could change part of the sentence to passive like this

The manufacturer promised (that) the product would be repaired or replaced if necessary within warranty period by the manufacturer.

This sounds more natural than the fully passive version of sentence 2.

Answer 2

You can say:

The product will be repaired or replaced as promised by the manufacturer, if necessary within warranty period.

Answer 3

Here's another variant, using an existential "if" as mentioned by @Kris in the comments:

It was promised by the manufacturer that they would repair or replace the product if necessary within [the] warranty period.

The "It was promised by x that y" structure is attested in other sources, although it is uncommon. Instead of using would, other modal verbs like should, will or shall might be used depending on the exact meaning, formality level, or variety of English being used.

Examples:

It was promised by the Lord to Abraham over 2 two thousand years ago that Christ should descend from him through the lineage of David.

from Coming to Know God, Yourself and Others: Easy Read for Teenagers and new Christians, by Franklyn Rolle

It was promised by the master, that, in thirteen hours, we should be in Avignon

from Switzerland, the South of France, and the Pyrenees, by Derwent Conway

In general, though, sentences of the form "x promises to y," where x is a noun phrase and y is a verb phrase, can't be transformed into passive forms without changing the wording.

For example, we can't say:

To repair or replace the product if necessary within [the] warranty period was promised.

There's some discussion of this fact in a book I found online, Linguistic Inquiry, but unfortunately I cannot access it so I don't know what it says, or even who the author of the relevant article is. I'll try to get access through my library and update this answer if I do.