Troubleshooting

Symptom → fix, in the order you'll hit them.

Red light on, no green activity light

Symptom: power LED lit, ACT LED dark, nothing on screen.

Cause: the Pi can't read the SD card — bad flash, bad card, or bad card slot contact.

Fix: reflash with Raspberry Pi Imager (verify the write when it offers). Try a different card — preferably a name-brand one. Reseat the card firmly; the click matters.

Boots, then random crashes or reboots

Symptom: works for minutes or hours, then dies — especially under load.

Cause: 9 times out of 10, the power supply. Phone chargers and long thin cables can't hold 5V when the CPU spikes.

Fix: the official supply for your model (5V/5A Pi 5, 5V/3A Pi 4). Check for the lightning-bolt (undervoltage) icon, or vcgencmd get_throttled — a nonzero result means it's happened since boot:

vcgencmd get_throttled

throttled=0x0

0x0 is clean. Anything else, fix the power first and suspect everything else second.

Wi-Fi connects, then drops

Symptom: SSH works, then hangs; the Pi vanishes from the network intermittently.

Cause: power management putting the Wi-Fi chip to sleep, or (again) marginal power.

Fix: disable Wi-Fi power management:

sudo iw dev wlan0 set power_save off

Make it permanent via a udev rule or a systemd unit. If it still drops, suspect power before drivers.

"No space left" with space to spare

Symptom: df -h shows free space, but writes fail.

Cause: the filesystem was never expanded to fill the card — or you're out of inodes (millions of tiny files).

Fix: sudo raspi-config → Advanced → Expand Filesystem (Imager does this automatically now, but old cards bite). For inodes: df -i — if it's 100% full, find the culprit with du.

Runs hot and throttles

Symptom: sluggish under sustained load; vcgencmd measure_temp reads 80°C+.

Cause: the Pi 5 especially needs active cooling for sustained work; the Pi 4 mostly doesn't, until you hammer it.

Fix: a fan (the official Pi 5 active cooler is $5 and excellent) or at least a heatsink case. Check current temp anytime:

vcgencmd measure_temp

temp=57.5'C

Design for your workload: a Pi-hole never breaks a sweat; a Pi running Ollama will cook without airflow.

SD card corrupted after power loss

Symptom: won't boot after an outage; filesystem errors.

Cause: SD cards hate sudden power loss mid-write.

Fix: short term, reflash from your backup (you have one — see Pi OS Essentials). Long term: a UPS for the Pi, or boot from NVMe/USB-SSD which tolerates abuse far better.

SSH "connection refused" on a fresh flash

Symptom: ping works, SSH doesn't.

Cause: SSH wasn't enabled in the Imager settings.

Fix: power off, put the card back in your computer, and create an empty file named ssh (no extension) in the boot partition — or just reflash with the gear-icon SSH option checked. The file method works without reflashing.

apt is painfully slow

Symptom: updates crawl.

Cause: the default mirror is far away, or the SD card is slow at random writes (all of them are, relatively).

Fix: sudo raspi-config → Localisation → pick a nearby mirror. And accept that apt upgrade on SD storage takes a while — it's the card, not your network.