Receptacle Tester Reads Open Ground But Tests Good Decision Tree

Why this matters

A receptacle tester showing open-ground at a receptacle that reads 0 ohms ground-to-neutral on a continuity meter is a common scenario that has two very different root causes, and conflating them is dangerous. The good case is an oxidized but intact EGC making poor contact with the receptacle's ground blade only under load; the deadly case is a bootleg ground (a hidden neutral-to-ground jumper at the device) that fools every standard plug-in tester into reading proper ground while leaving the user with no real protection. NEC 2023 Article 250.130 is strict about EGC integrity and prohibits the bootleg fix. The decision tree separates the two.

Symptom presentation

The tech sees: plug-in tester shows the two-amber "open ground" pattern; pull the receptacle, jumper the tester clips between ground and neutral, get a 0-ohm reading; reinstall and the tester still reads open ground; or the inverse, the tester reads "correct" but the device is suspicious because the box has no EGC visible.

Document the install. Knob-and-tube, 2-wire NM (pre-1965), BX armored cable, and conduit installations all present unique EGC situations. K&T has no EGC by design. 2-wire NM has no EGC. BX uses the armor as EGC and may or may not be continuous. EMT and rigid conduit use the raceway as EGC and may or may not be bonded at every junction.

Quick checks

Pull the receptacle and physically inspect. Look for an actual green-or-bare conductor landed on the ground screw. If present, verify it leads to a junction box where it continues to the panel via metallic raceway, armored sheath, or a separate EGC conductor. If absent, you have a 2-wire installation; the next decision is whether the install is grandfathered (allowed under 406.4(D)) or whether someone has cheated.

The bootleg test: with the receptacle pulled, lift the neutral conductor off its terminal. If the ground at the device shows a small jumper screwed to where the neutral was, you have found the bootleg. NEC 250.142(B) explicitly prohibits this; the bootleg defeats every plug-in tester and presents an energized chassis under any fault condition.

The reverse-bootleg test: a 2-wire receptacle with a "ground" that is actually a hot under fault. Read between the ground terminal and a known earth reference (a copper pipe, a properly-grounded receptacle elsewhere). Any voltage on the ground terminal is a bootleg or a damaged neutral feeding back.

Isolation tree

If the ground appears intact at the device but plug-in tester reads open: the EGC contact at the receptacle blade is the next suspect. Push-in (back-stab) terminals are notorious for losing ground continuity over years of plug insertions. Read continuity between the receptacle's ground screw and the receptacle's ground blade with the receptacle out of the wall; failure here is the receptacle and it is replaced.

If continuity to the panel is bad upstream, the next test is at the panel itself. Read EGC continuity from the receptacle blade to the panel ground bar; a healthy 14 AWG run should read under 1 ohm. Above 5 ohms is a marginal connection somewhere in the run; above 10 ohms with no continuity below means a broken EGC.

For conduit-as-EGC installs (EMT, rigid), the most common failure is a loose set-screw connector at a junction box. Megger-test continuity from each junction back to the panel; the box with no continuity to upstream is the broken section.

For knob-and-tube or 2-wire NM, you cannot create an EGC out of nothing. NEC 406.4(D) allows three options for an existing 2-wire receptacle: replace with 2-prong, replace with GFCI and label "no equipment ground" per 406.4(D)(2)(c), or pull a new EGC. Bootlegging is not an option.

Confirming diagnosis

Use a Suretest CT or Ideal SureTest 61-165 type EGC impedance tester, not just a plug-in. These run a controlled current through the EGC and report impedance; a healthy 14 AWG run reads under 1 ohm. A bootleg ground will read a different signature (low impedance to neutral but high to actual earth reference) that the SureTest can flag.

For deadliest case (the bootleg), confirm with the AC voltage probe between the ground terminal and a real earth electrode. A bootleg under normal conditions reads zero (because the bootleg goes to neutral which is near zero) but under any hot-to-chassis fault on the device or appliance, the bootleg passes the fault to the user.

Remediation

Replace the receptacle if continuity through the device is bad. Re-make the splice at the upstream junction box if continuity through the cable is bad. For 2-wire installs, follow NEC 406.4(D): 2-prong replacement, or GFCI with labeling, or pull a new EGC. Never bootleg; if you find a bootleg in an existing installation, remove it, document, and present the homeowner with the 406.4(D) options.

For knob-and-tube, NEC 394.10(2) permits extending K&T only in limited cases; in most jurisdictions and most insurance contexts, the right answer is to retire the K&T circuit and run a new branch on NM cable with EGC.

References

• NEC 2023 Article 250.130 (EGC continuity), 250.142(B) (prohibition of neutral-to-ground bond on load side), 406.4(D) (existing 2-wire receptacle replacement options), 394.10 (knob-and-tube extension limits)
• UL 498 (Attachment Plugs and Receptacles; ground blade contact requirements)
• NFPA 70B 2023 Chapter 11 (testing EGC integrity in existing installations)
• Ideal SureTest 61-165 Operating Manual (bootleg ground detection signature)