You can't just reject it after 4 months - you have to let the dealer attempt a repair first, but ...the dealer is only liable for faults that existed at the time of sale. Arguably after 4.5 months and maybe 4 or 5k miles running OK, this fault was not present at the time of sale (unless it's the same injector) - although it could have been 'developing' unseen. Hard to prove. Until you've had it diagnosed and the fault confirmed you don't really know. A court case might go in your favour, but it's a lot of effort.
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