Honda Civic FC5 - Warranty Dispute on Lighter Repair

I took my Honda Civic FC5 to the service center for a lighter socket issue. They refused to repair it, claiming that the car maintenance had not been completed, thereby voiding the warranty. I take care of the car's maintenance ahead of schedule. However, they are providing this note because I did not perform the service at their authorized location. When I contacted Honda headquarters, they claimed that it was their decision, in addition to the authorized service, and they showed no interest. They casually mentioned that I could take legal action. It's basically a "We won't help; do whatever complaints you want" attitude. While they claim my six-year conditional warranty is void, they have yet to produce any documentation proving that the standard three-year warranty has been voided. There is no service in terms of customer satisfaction. They're only looking for unjust gains. I will not let this go without seeking justice.