it randomly goes offline on the network and disappears from hp smart app until i restart it.

Diagnosis

The HP OfficeJet Pro 8135e suffers from a network handshake failure specifically tied to the "Auto-Off" and "Deep Sleep" power states. When the printer enters its lowest power consumption mode, the wireless NIC (Network Interface Card) enters a dormant state to save energy. Due to a firmware instability in the 8100 series, the printer fails to send a "wake-up" packet to the router, causing the router to drop the IP assignment. Because the HP Smart app relies on a constant heartbeat signal via the HP Cloud service, the app marks the printer as "Offline" the moment the lease expires. A restart forces a new DHCP request and re-establishes the handshake, but the cycle repeats as soon as the printer hits its idle timer. You can confirm this is the issue if the printer works perfectly for 30 minutes after a reboot but disappears once it has been idle for an hour.

How to Fix It

To resolve this, you must force the printer to maintain a persistent network heartbeat and stabilize the IP address. Follow these exact steps:

1. Disable Aggressive Sleep: On the printer's control panel, swipe down the dashboard and tap the Gear icon (Settings). Navigate to Preferences > Power Management > Sleep Mode. Change the setting from "Default" to "Never" or the maximum available time limit.
2. Turn Off Auto-Off: In the same Power Management menu, locate Auto-Off. Ensure this is toggled to Off. This prevents the printer from completely severing the power to the wireless card.
3. Assign a Static IP: Open a web browser on your computer and type the printer's current IP address (found under Network Summary in Settings) into the URL bar to open the Embedded Web Server (EWS). Go to the Network tab > IPv4 Configuration. Change the IP address assignment from "Automatic/DHCP" to "Manual/Static." This prevents the printer from disappearing when the router rotates IP addresses.
4. Force Firmware Update: On the printer touchscreen, go to Settings > Printer Maintenance > Printer Update. Select Check for Updates Now. If it says it is up to date, restart the printer and check again; often, the 8135e requires two consecutive checks to trigger a pending "Critical" update.

If That Didn't Work

• Switch to 2.4GHz Band: The 8135e often struggles with "Band Steering" on modern Mesh Wi-Fi systems where 2.4GHz and 5GHz share the same SSID. Access your router settings and create a separate guest network exclusively on the 2.4GHz band. Connect the printer to this dedicated band to eliminate signal dropping during frequency hops.
• Disable IPv6: In the printer's Embedded Web Server (EWS) under the Network tab, locate the IPv6 settings and uncheck "Enable IPv6." Many home routers struggle with IPv6 routing for HP peripherals, leading to the "Offline" status in the HP Smart app while the printer remains physically connected.
• Reset Network Defaults: On the printer screen, go to Settings > Network Setup > Restore Network Defaults. This clears the cached DNS and gateway settings. After the reset, reconnect the printer using the HP Smart app from scratch to rebuild the cloud handshake.
• Disable "Fast Startup" on PC: If the printer appears offline specifically after your computer wakes from sleep, disable "Fast Startup" in Windows Power Options. This forces the PC to re-initialize the printer driver and network port upon wake-up.

Heads Up

• Avoid USB-to-WiFi Mixing: Do not keep the printer connected via USB while simultaneously trying to use the HP Smart app over WiFi; this can cause a driver conflict that triggers the "Offline" error.
• Router Placement: Ensure the printer is at least 3 feet away from the router. Placing it too close (under 2 feet) can cause "signal clipping," leading to intermittent drops.
• HP+ Subscription: If you are using the "e" series features (Instant Ink), ensure your account is active. A lapsed subscription or payment failure can occasionally trigger a cloud-side disconnection that mimics a network failure.
• Firmware Warning: Always ensure the printer has a full tray of paper before starting a firmware update. If the printer pauses for a "Paper Out" error during a flash, it can occasionally brick the network card.