Yes the grammar checking is correct in this case. It could be correct in some cases, but not in the particular case you have in your example, because it is a compound sentence. When you are using a conjunctive adverb to combine two sentences, you should proceed the conjunctive adverb with a semi-colon, as explained on [grammarerrors.com][1]: > Sometimes writers use words such as however, furthermore, and > therefore (these are called conjunctive adverbs) in place of > coordinating conjunctions to combine two sentences into one. This is > where a punctuation problem often arises. The mistake writers make is > to incorrectly place a comma in place of a semicolon before > conjunctive adverbs, as illustrated in the sentence below: > > Example 2: The festival was to be held today, however, it was canceled > due to the rainy weather. (comma preceding the conjunctive adverb – > INCORRECT. Note: The comma following the conjunctive adverb is > perfectly correct and should be left as is.) This only applies to compound sentences though. As further explained at the above linked site: > The sporting events, however, continued despite the weather. Is perfectly fine, as it is not a compound sentence. [1]: http://www.grammarerrors.com/punctuation/commas-with-conjunctive-adverbs-however-furthermore-etc/