Klein Tools
Klein Tools MM420
Documentation-based review, without independent bench testing. Scores are editorial; prices are reference values. Confirm the seller’s current offer.
A practical general electrical meter with TRMS and temperature measurement. Its 4,000-count display suits routine checks; choose a different instrument when high-resolution logging or a higher measurement category is required.
Specifications
| Display Count | 4,000 |
| CAT Rating | CAT III 600 V |
| DC Voltage | Up to 600 V |
| AC Voltage | Up to 600 V; specified 50–400 Hz |
| Current | µA / mA / A ranges, up to 10 A; duty limits apply |
Features
What We Like
- ✓TRMS and automatic/manual ranging
- ✓Dedicated µA/mA ranges; temperature measurement
Limitations
- ✗4,000 counts; no documented PC logging
- ✗Magnetic hanger is optional; included thermocouple is limited to below 180 °C
Overview
MM420 is a sensible general electrical meter for buyers who want TRMS, automatic ranging and temperature measurement without paying for a premium industrial instrument. It replaces our MM400 recommendation as a new model with its own review, not as a relabeled MM400.
Review basis: manufacturer documentation and a dated exact-model Amazon identity check. No hands-on measurements or ownership experience are claimed.
Capability and resolution
Klein specifies 4,000 counts, CAT III 600 V, AC/DC voltage to 600 V and current to 10 A. Dedicated µA/mA ranges are present. Resistance reaches 50 MΩ; capacitance, frequency, diode and continuity functions are included. These functions make it a practical mixed-use tool, although display counts alone do not establish precision.
Limits that affect the decision
AC accuracy is specified over 50–400 Hz; TRMS does not mean accurate on arbitrary high-frequency waveforms. Above 6 A, follow the manual’s short measurement and recovery duty limits. The meter can measure higher temperatures with a suitable probe, but the included thermocouple is limited to below 180 °C. The magnetic hanger is optional.
When to spend more
Choose another meter for PC logging, greater resolution, or a higher required measurement category. No NCV function is documented for MM420. Read the exact meter and lead markings and use the correct current terminal; a category label does not make every circuit safe to measure.
Sources and review basis
Documentation reviewed 2026-11-01. Manufacturer specifications are distinct from Amazon price and availability checks. No independent bench testing is claimed.
Frequently Asked Questions
Was this instrument independently bench-tested by HobbyistScope?
Klein Tools MM420
$64.6 — CAT III 600 V · 4,000 count