Apple has blamed “external physical damage” for causing a handful of iPhones to explode or catch fire in China and insisted that its handsets posed no safety problem.The company also denied being slow to respond, after the state-run Shanghai Consumer Council had urged it to address consumer complaints.The watchdog’s report quoted one woman as saying her iPhone 6s Plus exploded in August, shattering the screen and leaving the battery and back of the phone blackened.Apple last month offered to change iPhone 6s batteries for Chinese users who complained of the sudden shutdowns, but said the problem did not constitute a safety issue.

To address the issue, Apple started offering free replacement batteries to the affected customers. But as the problem continues to appear on more devices, Apple is preparing a diagnostic tool to gather more information and figure out if it can be resolved with a forthcoming iOS update.

The diagnostic tool will presumably come in the form of iOS 10.2 next week. Apple has said that the issue stems from some batteries being overexposed to “controlled ambient air” during manufacturing.

If you are affected by unexpected battery shutdowns, you will need to make an appointment at your local Apple store to verify that your iPhone 6s is eligible for the free battery replacement. An Apple store employee will also check for a broken screen, scratches, or dents that would require additional repairs not covered by the replacement program.

In most cases, the replacement battery will have to be ordered and might take up to seven days to arrive. In the meantime, you’ll be instructed to do an iPhone backup and to turn off Find My iPhone. Once your replacement battery has arrived, you will need to drop-off your iPhone 6s at the Apple store. Then, it will take anywhere from one or two hours for the battery to be replaced.

According to the work authorization receipt, a battery replacement kit is valued at $79, but it is free for customers who qualify for the program. You can check here to see if your iPhone 6s might be eligible for a free battery replacement.