Intelligent feedback on learners’ full written sentence productions requires the use of Natural Language Processing (NLP) tools and, in particular, of a diagnosis system. Most syntactic parsers, on which grammar checkers are based, are designed to parse grammatical sentences and/or native speaker productions. They are therefore not necessarily suitable for language learners. In this paper, we concentrate on the transformation of a French syntactic parser into a grammar checker geared towards intermediate to advanced learners of French. Several techniques are envisaged to allow the parser to handle ill-formed input, including constraint relaxation. By the very nature of this technique, parsers can generate complete analyses for ungrammatical sentences. Proper labelling of where the analysis has been able to proceed thanks to a specific constraint relaxation forms the basis of the error diagnosis. Parsers with relaxed constraints tend to produce more complete, although incorrect, analyses for grammatical sentences, and several complete analyses for ungrammatical sentences. This increased number of analyses per sentence has one major drawback: it slows down the system and requires more memory. An experiment was conducted to observe the behaviour of our parser in the context of constraint relaxation. Three specific constraints, agreement in number, gender, and person, were selected and relaxed in different combinations. A learner corpus was parsed with each combination. The evolution of the number of correct diagnoses and of parsing speed, among other factors, were monitored. We then evaluated, by comparing the results, whether large scale constraint relaxation is a viable option to transform our syntactic parser into an efficient grammar checker for CALL.