Receptacle Tester Fault Pattern Decision Tree

Why this matters

A three-light plug-in receptacle tester (Klein RT250, Ideal 61-501, Sperry GFI6302) gives a quick at-the-receptacle indication of common wiring faults: open ground, open neutral, open hot, hot-ground reverse, and hot-neutral reverse. The light pattern is industry standardized but the field interpretation is not. Each pattern has more than one possible cause, and the tester cannot distinguish certain dangerous conditions from harmless ones. This tree maps each pattern to the cause families and the next step for verification before any termination is opened.

The standard light patterns

A three-light tester uses two yellow lights and one red light arranged left to right with the plug ground pin down:

• Two yellow lit (left and middle), red off: Correct wiring
• One yellow lit (right) only: Open ground
• One yellow lit (middle) only: Open neutral
• All three off: Open hot
• Red and one yellow (right): Hot-ground reverse
• Two yellow lit (left and right), no red: Hot-neutral reverse (on some tester models the pattern differs; check the legend on the tester body)

Modern testers may add a GFCI test button that trips the upstream GFCI by injecting a 5 mA imbalance current.

Verification before opening a receptacle

Before pulling the cover plate:

1. Test the tester on a known-good receptacle. A failed tester reads identical patterns regardless of true wiring.
2. Verify the suspect pattern repeats consistently. A single-time anomaly may be the tester itself; plug in three times to confirm.
3. Check adjacent receptacles on the same circuit. A whole-circuit open neutral presents on the first receptacle in the run and propagates downstream.

Pattern 1: Open ground

The receptacle has hot and neutral but no equipment grounding conductor.

Causes in field order:

1. Two-wire wiring (pre-1965 NM cable or ungrounded knob-and-tube), no ground available. NEC 406.4(D)(2) permits replacement with either a GFCI receptacle (label as "No Equipment Ground") or a three-prong receptacle protected by an upstream GFCI breaker.
2. Ground conductor present but not landed at the receptacle. Open the box and inspect; common with novice work.
3. Ground broken in a series of receptacles where one upstream receptacle did not pigtail the ground correctly (back-stab termination relies on pass-through; a single break opens downstream).
4. Metal box used as the ground path with a wire connector missing between the box screw and the receptacle green; NEC 250.146(A) requires a bonding jumper unless self-grounding receptacles are used.

Action: confirm visually before declaring two-wire. Many open-ground reports turn out to be a disconnected green wire taped back inside the box.

Pattern 2: Open neutral

The receptacle has hot and ground but the neutral path is broken. A receptacle on an open neutral will deliver line voltage if a load is connected because the load current finds a path through the ground conductor, which is a shock hazard.

Causes:

1. Loose back-stab termination at an upstream receptacle on the same circuit
2. Neutral wire cut or broken at a wire connector inside a junction box
3. Multi-wire branch circuit with shared neutral broken; the open neutral can put 240 V across both circuit loads
4. Service neutral open at the main panel (whole-house symptom; lights brighten and dim with load shifts)

Action: kill the breaker before opening any boxes. An open neutral with hot present means metal box bodies and ground conductors may be at full line voltage referenced to true earth, depending on the connected load.

Pattern 3: Open hot

The receptacle has neutral and ground but no hot. May be:

1. Tripped breaker
2. Tripped GFCI upstream
3. Tripped AFCI upstream (NEC 210.12 covers AFCI on most 120 V branch circuits in dwellings)
4. Loose hot wire at upstream receptacle or at the breaker
5. Failed switch (when the receptacle is switched, common in some bedroom configurations)

Action: check the panel first. The vast majority of open-hot calls trace to a tripped breaker that the homeowner did not notice was tripped because tripped AFCIs and GFCIs often appear visually nearly identical to "on."

Pattern 4: Hot-ground reverse

The hot and ground conductors are swapped at a wire connector or termination. This is dangerous: the box, the receptacle yoke, all connected appliance frames, and the grounding conductor itself are at line voltage.

Causes:

1. Receptacle wired with hot on the green screw and ground on the brass screw; common on field rewires by inexperienced workers
2. Wire connector miswired in an upstream box putting ground conductor on hot terminal downstream

Action: kill the breaker immediately. Do not let the customer continue using the receptacle. Open the upstream and current boxes to find the swap point and correct it.

Pattern 5: Hot-neutral reverse

Hot is landed on the silver (neutral) screw and neutral is landed on the brass (hot) screw. Tools and lamps still operate; the equipment grounding path is intact; the user may not notice. Reverse polarity is dangerous because internal switching in lamps and small appliances leaves the high side energized when the appliance is "off."

Causes:

1. Receptacle wired backwards at the receptacle in question
2. Upstream NM cable jacket markings ignored during termination

Action: open the receptacle and re-land conductors per NEC 200.11 (white neutral to silver screw, hot to brass screw, ground to green screw).

What the tester cannot detect

• Bootleg ground (hot or neutral jumpered to the ground screw at the receptacle). The tester reads "correct" but no actual grounding electrode connection exists. A receptacle tester with GFCI button can flag this on a GFCI-protected receptacle but not on a non-GFCI receptacle.
• Reversed neutral and ground at the panel main bonding jumper location (the tester reads correct from the load side).
• Loose connections that test correctly on the light pattern but fail under load. Use a clamp meter on hot conductors under realistic load to confirm continuity quality.
• Shared neutral conductor on a multi-wire branch circuit. The tester does not detect that the circuit is multi-wire; an open neutral on a MWBC is a serious shock and equipment hazard.

Recommended downstream verification

After fixing any wiring fault:

1. Re-test with the receptacle tester
2. Use a multimeter to verify hot-to-neutral 120 V (+/- 6 V from nominal per ANSI C84.1)
3. Verify hot-to-ground 120 V (matches hot-to-neutral within 2 V)
4. Verify neutral-to-ground under 2 V (under no load) and under 5 V (under typical load)
5. Test GFCI receptacles with the tester's GFCI button and the receptacle's own TEST button; both must trip the receptacle

References

• NEC 2023 Article 250 Grounding and Bonding
• NEC 2023 Section 406.4(D) Replacements (covers ungrounded receptacle replacement)
• NEC 2023 Section 200.11 Polarity of Connections
• ANSI C84.1-2020 American National Standard for Electric Power Systems and Equipment Voltage Ratings
• OSHA 29 CFR 1910.304 Wiring Design and Protection