This is a tricky problem, and it sounds like you've already done some good troubleshooting. Here's a breakdown of potential causes and how to investigate them

Understanding the Interaction
AFCI vs. GFCI

An AFCI (Arc Fault Circuit Interrupter) protects against arcing faults (dangerous sparks). A GFCI (Ground Fault Circuit Interrupter) protects against ground faults (current leaking to ground). While they serve different purposes, it's not uncommon for them to interact, especially when wired downstream of each other. The GFCI tester introduces a very small current to ground, which can trigger the AFCI if there's already some other subtle issue.

Sensitivity

AFCIs are designed to be sensitive, which is good for safety, but it can also lead to nuisance tripping. GFCI's are also very sensitive.

Downstream Issues

The problem could be at any of the three downstream outlets, or even in the wiring between them.

Troubleshooting Steps

Here's a systematic approach to isolate the problem:1. Double-Check All Connections

Go back and meticulously check every connection at the GFCI receptacle, the three downstream outlets, and the AFCI breaker itself. Look for:

Loose wires

Even a slightly loose connection can cause arcing.

Nicked insulation

A nick in the insulation can allow current to leak to ground or to other conductors. Pay special attention to where wires enter and exit boxes.

Wires touching the box

If the box is metal and a wire is touching it due to a missing connector or damaged insulation, this can create a ground fault.

Proper grounding

Ensure the ground wire is properly connected at all locations and that all metal boxes are grounded.

Use a Torque Screwdriver

Ensure all screw terminals are tightened to the manufacturer's recommended torque. Over-tightening or under-tightening can cause issues.2.

Disconnect the Downstream Outlets (One by One)



The Key Test

This is the most important step. Disconnect the wiring at the first downstream outlet, capping off all wires (hot, neutral, ground) with wire nuts. Temporarily remove the outlet entirely from the circuit.

Test the GFCI again. If the AFCI doesn't trip, then the problem is somewhere downstream of that first outlet. Repeat with the second and third outlets, one at a time, until the problem is isolated. Each time, re-test the GFCI after disconnecting each outlet. If it trips even with ALL downstream outlets disconnected

The problem is likely in the wiring between the GFCI receptacle and the AFCI breaker, or the GFCI receptacle itself is faulty.3.

Inspect the Wiring Between the GFCI and the First Downstream Outlet (If Downstream)

If you've isolated the problem to a specific downstream run, carefully inspect the wiring in that run. Look for:

Damaged cable

Nicks, cuts, or signs of rodent damage.

Wiring routed near pipes or other metal objects

Check for potential shorts.

Improperly stapled wiring

Staples driven too tightly can damage insulation.4.

Test the GFCI Receptacle Itself

Even new GFCIs can be faulty. Replace the GFCI receptacle with a brand new one of good quality (Leviton, Eaton, or Hubbell are generally recommended). Test it before connecting any downstream outlets. If the AFCI still trips with a known good GFCI and no downstream outlets, the issue is definitely upstream of the GFCI.5.

Check the AFCI Breaker



AFCI Tester

Use a dedicated AFCI tester (not just a GFCI tester) to test the AFCI breaker itself. These testers simulate an arcing fault. If the AFCI breaker doesn't trip with the AFCI tester, the breaker may be faulty and need to be replaced.

Visual Inspection

Look for any signs of damage or overheating on the breaker.6.

Neutral-Ground Bonding Issue



Important

A major cause of GFCI/AFCI tripping is a neutral-ground bond downstream of the GFCI. There should only be one neutral-ground bond in the entire electrical system, and that's at the main service panel.

How to Check

Use a multimeter. With the circuit de-energized (breaker OFF), measure the resistance between the neutral wire and the ground wire at each outlet. You should see infinite resistance (an open circuit). If you see a low resistance reading (close to zero ohms), there's a neutral-ground bond somewhere.7.

Shared Neutral (Multi-Wire Branch Circuit Problem)

If this circuit shares a neutral wire with another circuit (a multi-wire branch circuit), it can cause problems with AFCIs and GFCIs. This is a more complex issue, and you might need an electrician to diagnose it.8.

Appliance Issues

While less likely, a faulty appliance plugged into one of the outlets could be introducing a small amount of ground fault current that's enough to trigger the AFCI when the GFCI is tested.

Important Safety Notes



Turn off the Breaker

Always turn off the circuit breaker before working on any electrical wiring.

Test After Every Step

Test the circuit after every change you make to ensure you're not introducing new problems.

If You're Not Comfortable

If you're not completely comfortable working with electrical wiring, do not proceed. Call a qualified electrician. Electrical work can be dangerous.

Licensed Electrician

For final verification and for code compliance, consult a licensed electrician.

In Summary

The most likely culprits are:

Loose or Damaged Wiring

Meticulously check all connections.

Neutral-Ground Bonding Issue

Check for downstream neutral-ground bonds.

Faulty GFCI Receptacle

Try replacing it with a known-good one.

Faulty AFCI Breaker

Test it with an AFCI tester.*

Issue with a Downstream Outlet or Wiring: Systematically disconnect them to isolate the problem.Good luck troubleshooting! Take your time, be methodical, and prioritize safety. I hope this helps! Flag for review