The Klein Tools MM420 is a True RMS auto-ranging digital multimeter rated CAT III 600V. It measures AC/DC voltage up to 600V, AC/DC current up to 10A, resistance to 50 MΩ, capacitance up to 400 µF, frequency, duty-cycle, and temperature via thermocouple. Lead Alert LEDs illuminate to indicate correct lead placement for added safety. The 4000-count backlit LCD and 6.6-foot drop protection suit daily field use. Data hold, auto power-off, and min/max recording are included. Backed by Klein Tools' 1-year limited warranty.
What does True RMS mean and why does it matter?
True RMS measures the actual power in non-sinusoidal waveforms accurately. Without it, readings on motors, variable-speed drives, or electronic ballasts can be off by 10–40%. The MM420 gives correct readings on those loads without manual correction — average-sensing meters like the MM325 do not.
Does the MM420 include temperature measurement?
Yes — a thermocouple probe is included. Reviewers use it for motor housing checks, HVAC diagnostics, and outlet temperature verification alongside standard voltage and current measurements.
How durable is the MM420?
Klein bench-tests it to survive a 3-foot drop from a workbench. Multiple reviewers call out the firm dial action and solid button feel as standout build quality for the price — one reviewer noted it replaced a meter used for many years in tough conditions.
What's the difference between the MM420 and MM325?
MM420 is True RMS and auto-ranging; MM325 is manual-ranging and average-sensing. For circuits with motors, VFDs, or non-sinusoidal loads, the MM420 reads correctly where the MM325 would give a misleading result.
Is the display readable in dark panels?
Yes — the backlit LCD is consistently described by reviewers as clear and easy to read in low-light conditions. The display layout is called out as intuitive and well-organized.
What measurements can the MM420 take?
AC/DC voltage up to 600V, AC/DC current up to 10A, resistance up to 50MΩ, capacitance, frequency, continuity with audible beep, diode test, and temperature via the included thermocouple.
# Klein Tools® MM420 Digital Multimeter, Auto-Ranging, 600V **Model:** `MM420` **Brand:** Klein Tools **Category:** Multimeter > The Klein Tools MM420 is a True RMS auto-ranging digital multimeter rated CAT III 600V. It measures AC/DC voltage up to 600V, AC/DC current up to 10A, resistance to 50 MΩ, capacitance up to 400 µF, frequency, duty-cycle, and temperature via thermocouple. Lead Alert LEDs illuminate to indicate correct lead placement for added safety. The 4000-count backlit LCD and 6.6-foot drop protection suit daily field use. Data hold, auto power-off, and min/max recording are included. Backed by Klein Tools' 1-year limited warranty. ## Specifications | Attribute | Value | |---|---| | Product Type | Digital Multimeter | | True Rms | Yes | | Auto Ranging | Yes | | Voltage Range Ac | 600V AC | | Voltage Range Dc | 600V DC | | Current Range Ac | 10A AC | | Current Range Dc | 10A DC | | Resistance Range | 50 MΩ | | Capacitance Range | 400 µF | | Frequency Range | 1 Hz – 50 KHz | | Temperature Range | -40–1832°F (-40–1000°C) | | Low Impedance Loz | No | | Nist Calibrated | No | | Display Type | LCD | | Display Count | 4,000 | | Cat Rating | CAT III 600V | | Battery Type | 2 x AAA | | Overall Length In | 6.41 | | Assembled Weight Oz | 8.2 | | Barcode (UPC) | 092644693748 | | Warranty | 1-year limited | ## What's Included - Multimeter - Test leads with CAT III/CAT IV safety caps - Thermocouple - Instructions - 2 x AAA batteries Source: https://www.jaspecs.com/klein-tools/multimeter/mm420-klein-tools-digital-multimeter-auto-ranging-600v/