When I look in the system journal I see the message wlan0: DHCP lease expired. Could this explain why my Raspberry Pi stops working and I'm unable to connect via SSH? Is there a solution or steps I could take to further diagnose the problem?
Yes, it is possible that your Raspberry Pi is stopping working and you are unable to connect via SSH because the DHCP lease has expired. When a DHCP lease expires, the device loses its IP address and can no longer communicate with other devices on the network.

To diagnose the problem, you can try the following steps:

1. **Check the DHCP lease time.** You can do this by running the following command:

```
sudo cat /etc/dhcpcd.conf
```

Look for the line that says `dhcp_lease_time`. This is the amount of time that your Raspberry Pi will keep its IP address before it expires.

2. **Try renewing the DHCP lease.** You can do this by running the following command:

```
sudo dhclient -r wlan0
```

This will release the current IP address. Then, run the following command to renew the lease:

```
sudo dhclient wlan0
```

3. **Check the network connection.** You can do this by running the following command:

```
ifconfig
```

This will show you a list of all of the network interfaces on your Raspberry Pi, along with their IP addresses. Make sure that the `wlan0` interface has an IP address.

If you are still unable to connect to your Raspberry Pi via SSH, you can try connecting to it using a serial cable. Once you are connected, you can troubleshoot the problem further.

Here are some possible solutions to the problem:

* **Increase the DHCP lease time.** If your DHCP lease time is too short, your Raspberry Pi may lose its IP address before it has a chance to renew it. You can increase the lease time by editing the `/etc/dhcpcd.conf` file and changing the value of the `dhcp_lease_time` setting.
* **Set a static IP address.** If you have a reliable network, you can set a static IP address for your Raspberry Pi. This will prevent it from losing its IP address when the DHCP lease expires. To set a static IP address, edit the `/etc/dhcpcd.conf` file and add the following lines:

```
interface wlan0
static ip_address=192.168.1.100
static gateway=192.168.1.1
static subnet_mask=255.255.255.0
```

Replace `192.168.1.100` with the IP address that you want to assign to your Raspberry Pi, and replace `192.168.1.1` with the IP address of your router.

* **Update the firmware.** If you are using an older version of the Raspberry Pi firmware, it is possible that there is a bug that is causing the DHCP lease to expire prematurely. You can update the firmware by running the following command:

```
sudo rpi-update
```

If you are still having problems, you can try searching for help on the Raspberry Pi forums or Stack Exchange.
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