When my Viomi washing machine started showing E05 every time I turned it on, I had no idea whether it was still covered. I filed a service request through Viomi's official mini program and assumed the technician could check the serial number when he arrived.
The next day, the technician came. He scanned the barcode on the machine and told me the inlet solenoid valve and the water-level sensor had failed. He checked and said those two parts carried only a one-year warranty. Since I bought the machine in June 2021, the repair would have to be paid out of pocket: 280 yuan for one part, 140 yuan for the other, and 60 yuan for labor.
I asked whether those parts were really that expensive. He said the parts fee went to the manufacturer and that technicians did not make a penny from it, then offered to knock 10 yuan off the labor fee, down to 50. That was the moment I became suspicious. 
If the parts were passed through at cost and labor was his only income, why would he give up part of his own fee? The more likely explanation was that he was making money on the parts. 
I did not challenge him. I just played dumb and said, "I see."
He asked for a 300-yuan deposit by WeChat. When I transferred the money, I left a note: "Viomi washing machine repair parts fee." I wanted a record in case I had to argue later.
After he left, I was busy and did not check the warranty policy right away. My main concern was getting the machine fixed. Three days later, he came back, installed the new parts, and the machine still did not work. He even had to call a colleague to discuss how to repair it. By then, my patience was gone.
I contacted customer service and asked about the warranty. They said the whole machine had a one-year warranty, but important parts such as the inlet solenoid valve and the water-level sensor had a three-year warranty. I challenged the earlier quote. Customer service said they would pass the matter to the local service outlet, which would get back to me.
The next day, the outlet called and told me to talk to the technician directly. I lost my temper and said the outlet should resolve it and then report back to me. 
That response killed any remaining hope I had. I opened the 12315 mini program and filed a complaint against the seller.
Maybe not everyone has used 12315 to complain about a business. I want to emphasize this: if a business violates your legal rights as a consumer, do not be afraid of the hassle and do not back down. File a complaint when you need to. You can trust 12315.
My confidence came partly from a past experience. Around New Year's Day in 2018, some friends and I rented a car from Shenzhou Car Rental for a trip. A friend returned it for me. The company said there was a 100-yuan holiday surcharge, and because my friend was in a hurry, he did not argue on the spot. He told me the next day. I thought that was unacceptable: no one had mentioned the surcharge when we rented the car, and suddenly we were being charged extra. Did the rental agreement mean nothing? I called the 12315 hotline, explained what happened, and waited for a callback. A few days later, I got a call saying the business was willing to mediate. I said my demand was a refund of the overcharge, and they agreed. I got the money in about a week. That experience convinced me of 12315's deterrent effect on businesses.
Three days after I filed the complaint, someone from Viomi called. I stated two demands: honor the warranty policy and refund the 300 yuan the technician had collected from me. The person said they would communicate with the service outlet immediately. 
A day later, the technician called. He said he had remembered the warranty policy incorrectly, asked me to add him on WeChat, and refunded the money. I agreed.
Another day later, he came to repair the machine again. I happened to be working overtime, so my father supervised. It was finally fixed, and we tested it for several days. During that time, Viomi called again to ask whether the machine was working. I said it was, and that I would withdraw the complaint after a few more days if there were no problems. The person said okay and wished me a good life.
After a few days with no issues, I withdrew the complaint through the 12315 mini program. That was the end of it.
I still do not fully understand why businesses give in when 12315 is involved, but for ordinary people, having a platform they can trust to protect consumer rights is a good thing. 
I have heard that 12315's leverage over businesses varies by province and that some complaints fail. Even so, I do not think people should give up. If everyone gives up, no one will care. 