PRTG SNMP Error -2003: how to troubleshoot "No Response" without guessing

The reason -2003 feels random is that people troubleshoot it out of order. If you always work from the monitoring perspective inward, the cause falls out in minutes instead of an afternoon of guessing.

Start from the Probe, not your laptop

The most important question is: which Probe owns the failing sensor? PRTG SNMP monitoring originates from that Probe, so every test has to be performed from that same machine. Testing from your workstation proves nothing, because it takes a different path with different firewall rules and a different source IP.

A good first check is Paessler SNMP Tester on the Probe machine. Paessler specifically recommends a simple Read Device Uptime request to verify whether basic SNMP communication works at all. If uptime succeeds, the device is reachable over SNMP from that machine and you can stop blaming the network.

Troubleshoot in this exact order

PRTG's current SNMP documentation polls over UDP 161 and recommends confirming that SNMP traffic can pass bidirectionally between the monitoring system and the target. Bidirectional matters: the request going out is not enough, the response has to come back to the same Probe.

Ping working does not prove SNMP works

This is the single most common mistake. Ping and SNMP are different protocols taking different paths through different rules. One can succeed while the other is silently dropped.

Ping only proves ICMP connectivity. SNMP can still fail because of:

• Wrong community string or SNMPv3 credentials
• UDP 161 filtering
• SNMP agent disabled on the device
• An allowed-manager ACL that excludes the Probe
• Wrong SNMP version
• Target-side SNMP issues

Turn the order into decisions

Run these checks top to bottom from the Probe that owns the sensor. Each answer sends you to exactly one next move, so you never change two things at once.

If SNMP Tester works but the PRTG sensor fails

That is actually good news. It proves basic SNMP communication is working, so stop troubleshooting connectivity. Your focus should move to what is specific about the failing sensor:

• The specific OID being queried
• The interface index
• Sensor configuration
• Inherited credentials from the parent device or group
• GETBULK compatibility
• Multi-GET compatibility
• Vendor-specific SNMP behavior

For example, System Uptime may work while SNMP Traffic fails. At that point stop looking at basic connectivity and start looking at the specific interface table or OID being queried.

Watch for Probe migrations

A common enterprise version of -2003 appears right after moving monitoring from one Probe to another. The new Probe has a new source IP, and target ACLs do not know about it yet.

Old Probe IP   10.20.1.15   (allowed on the router)
New Probe IP   10.20.1.16   (NOT in the allowed-manager list)

Ping  10.20.1.16 -> device   works   (ICMP is open)
SNMP  10.20.1.16 -> device   -2003   (source IP rejected)

So after any Probe migration, always ask: has the SNMP source IP changed, and does the target still allow it? That one question resolves a large share of post-migration -2003 tickets, including the one in the thread above.