To debug the i2c connection, enable i2c tracing first to obtain a log of everything that is being sent/received from the sensor:
sudo sh -c "echo 1 >/sys/kernel/debug/tracing/events/i2c/enable"
sudo sh -c "adapter_nr==1 >/sys/kernel/debug/tracing/events/i2c/filter"
(To disable i2c tracing once you're done, run: sudo sh -c "echo 0 >/sys/kernel/debug/tracing/events/i2c/enable")
You'll be able to print the i2c log with sudo cat /sys/kernel/debug/tracing/trace.
For the i2c scanning command sudo i2cdetect -y 1 the log would look something like this:
...
I2CDetect-27461 [003] .... 2427.402606: i2c_read: i2c-1 #0 a=048 f=0001 l=1
I2CDetect-27460 [003] .... 2426.482952: i2c_reply: i2c-1 #0 a=048 f=0001 l=1 [15]
I2CDetect-27460 [003] .... 2426.482954: i2c_result: i2c-1 n=1 ret=1
I2CDetect-27460 [003] .... 2426.483009: i2c_read: i2c-1 #0 a=049 f=0001 l=1
I2CDetect-27460 [003] .... 2426.483218: i2c_result: i2c-1 n=0 ret=-121
...
For each exchange you'll find 2 or 3 lines, a read or write attempt, an optional reply for the response from the device and a result line with a return code and the size of the response.
I2CDetect-nnnnn [nnn] .... xxxx.xxxxxx: i2c_read: i2c-<adapter-number> #<message-array-index> a=<addr> l=<datalen>
I2CDetect-nnnnn [nnn] .... xxxx.xxxxxx: i2c_reply: i2c-<adapter-number> #<message-array-index> a=<addr> f=<flags> l=<datalen> [<data-transferred>]
I2CDetect-nnnnn [nnn] .... xxxx.xxxxxx: i2c_result: i2c-<adapter-number> n=<message-array-size> ret=<result>
Taking a look at the first example from i2cdetect, we can see that the read attempt for 0x48 was successful, since we got [15] back and the return code was 1.
With 0x49 we weren't so lucky, no one responded and the system logged this with an error code equal to -121 with 0 bytes transfered.
For more information on I2C:
http://www.circuitbasics.com/basics-of-the-i2c-communication-protocol/
https://electronics.stackexchange.com/questions/151413/how-to-display-i2c-address-in-hex
https://manpages.ubuntu.com/manpages/xenial/man8/i2cdetect.8.html