# Diag: agent service fails to start
**Symptom (exact):** `systemctl start datadog-agent` exits non-zero, or the container crash-loops. `agent status` never runs.
**Likely causes:** (a) yaml syntax error from a recent edit, (b) invalid option value (typo in a boolean, wrong type), (c) permissions on the config or log dirs, (d) port conflict from a second install.
**Confirm:**
1. Read the agent log from the failed start. The first error is usually a yaml parse error with a line number, or an unknown/invalid option name.
2. Validate the yaml with a parser (python yaml safe_load, or `yamllint`). Fix the reported line.
3. `git diff` the config if it is managed (it should be). The last change is the suspect; revert it and start.
4. Check for two installs: package + manual binary both trying to bind. `ss -ltnp` shows who holds the agent ports.
**Fix:** correct the yaml, fix ownership/perms on `/etc/datadog-agent` and the log dir, remove the duplicate install, start once.
**Verify:** the service stays up for 5 minutes (crash loops hide behind restart delays), `agent status` runs clean, forwarder accepts payloads. Then commit the fixed config so the next edit starts from known-good.