Klein Tools MM400 Multimeter: Specs, CAT III Rating and Status

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The Klein Tools MM400 is an auto-ranging digital multimeter for voltage, current, resistance, capacitance, frequency and temperature, rated CAT III 600 V. Searches for “klein mm400” usually come from people who own one, have one on a job, or are checking whether it covers a task. This guide sets out the figures Klein publishes for the MM400, explains what the CAT III rating means, and notes that Klein lists the model as no longer in production.

Quick answer

The MM400 measures AC and DC voltage to 600 V, AC and DC current to 10 A, and resistance to 40 megohms, according to Klein’s product page.
Klein lists CAT III 600 V and CE certification.
Klein’s product page says the MM400 is no longer in production and points to the MM420 as the newer model.
Replacement fuses are listed as 6×32 10 A 600 V and 5×20 500 mA 600 V.

What the MM400 measures

The MM400 is a handheld auto-ranging multimeter. Auto-ranging means the meter chooses the measurement range for you once you select the function, so you do not need to step through ranges by hand. That helps on routine checks, where you want a reading quickly and the expected value is unknown.

Klein’s page lists the following measurement limits. AC and DC voltage go to 600 V. AC and DC current go to 10 A. Resistance goes to 40 megohms. Capacitance goes to 200 microfarads. Frequency is listed from 1 Hz to 50 kHz. Temperature is listed from 0 to 1000 degrees F, which is -18 to 538 degrees C. Klein’s page does not list accuracy figures for these functions, so this guide does not give any.

The MM400 also covers diode and continuity tests. Those functions are standard on a multimeter of this type, but Klein’s product page as opened does not list their values, so check the manual for the exact test currents and thresholds.

Published specifications

Voltage: 600 V AC and 600 V DC.
Current: 10 A AC and 10 A DC.
Resistance: 40 megohms.
Capacitance: 200 microfarads.
Frequency: 1 Hz to 50 kHz.
Temperature range: 0 to 1000 degrees F (-18 to 538 degrees C).
Safety rating: CAT III 600 V, CE certified.
Power: two AAA batteries.
Weight: 8.2 oz (232 g).
Status: no longer in production, per Klein’s product page.

Klein’s replacement parts list gives two fuses. Part 69032 is a 6×32 mm, 10 A, 600 V fuse. Part 69033 is a 5×20 mm, 500 mA, 600 V fuse. The product page does not list a fuse rating in its specifications, so the parts list is the source for the fuse details.

What CAT III 600 V means for this meter

The CAT rating describes the electrical environment where the meter may be used, not only the voltage. Fluke’s guidance on measurement categories says that CAT III covers fixed installations inside a building, such as switchgear, polyphase motors and permanently wired equipment. The category sits alongside the voltage rating, so both have to match the circuit.

Fluke’s advice is to choose a meter rated for the highest category you could be working in. For the MM400, CAT III 600 V suits distribution-level work inside a building, such as checking a panel, a motor starter or a fixed outlet circuit. It does not cover the origin of an installation, where the supply enters the building. For that work you need a meter rated CAT IV, and the MM400 is not listed as CAT IV.

The meter rating also depends on the leads. Check that the leads, probe tips and fuses carry a rating at least as high as the meter’s CAT III 600 V rating. A meter with a good rating and worn leads is not safe on a high-energy circuit. Replace damaged leads before you start work.

Batteries, fuses and maintenance

The MM400 uses two AAA batteries. Replace them when the display dims or the readings become unstable. Keep spare batteries with the meter if you use it on a job site where a battery change is inconvenient.

Keep spare fuses of the correct type. The two fuses in the parts list are different sizes and current ratings, and the wrong one can give a false reading or fail to protect the meter. Use the fuse specified for the function. Do not substitute a fuse with a higher current rating than the one listed.

Check the case and the input jacks before each use. A cracked case, a loose jack or damaged insulation on the leads is a reason to stop and repair or replace the part before taking a reading.

Is the MM400 still a good choice?

Klein’s page says the MM400 is no longer in production. That matters most if you plan to buy one new. Used or distributor stock may still be on the market, but warranty support and replacement parts will depend on the seller and on Klein’s parts supply. If you are shopping now, compare the MM400’s published figures with the current Klein model that replaces it, and choose the model with the category, range and function set your work needs.

For a task that needs only basic voltage, current and resistance checks inside a building, the MM400 figures above are enough to decide whether it fits. For work above CAT III, or for work that needs a measurement the page does not list, look at other meters with the rating and function you need.

How we chose these figures

Every figure in this guide comes from Klein’s own MM400 product pages, on the US and UK sites, and the fuse part numbers come from the parts list on those pages. We did not use distributor listings, retailer pages, third-party manual hosts or forum posts for any specification. The CAT explanation comes from Fluke’s guidance on measurement categories.

What we could not verify

Klein’s product page does not state an accuracy figure for any function, so this guide does not give one. The page does not give a power figure beyond the battery type, does not state a warranty term, and does not list the diode or continuity test values. We did not open an official Klein PDF manual for the MM400 from an allowed source in this review, so check the instruction manual for your unit for the accuracy tables, test values and operating conditions before you rely on a specific number.

Frequently asked questions

What is the maximum voltage of the Klein MM400?

Klein lists 600 V AC and 600 V DC, with a CAT III 600 V safety rating.

Is the Klein MM400 CAT IV rated?

Klein’s page lists CAT III 600 V only. Do not use it for CAT IV work at the origin of an installation.

Is the Klein MM400 still in production?

Klein’s product page says it is no longer in production and points to the MM420.

For a wider view of Klein multimeters, see our Klein Tools multimeter lineup and the Fluke multimeter lineup.

Summary

The Klein Tools MM400 is an auto-ranging multimeter with listed ranges to 600 V AC and DC, 10 A AC and DC, 40 megohms and 200 microfarads, rated CAT III 600 V. Klein lists it as no longer in production. Check the category against your circuit, confirm the fuse parts, and look at the current Klein model if you are buying new.

Sources

Klein Tools, “MM400 – Digital Multimeter, Auto-Ranging, 600V”, product page, https://www.kleintools.com/product/MM400 , opened 2026-10-28.
Klein Tools UK, “MM400”, product page, https://www.kleintools.co.uk/product/MM400 , opened 2026-10-28.
Fluke, “Why use a CAT III rated meter in solar PV installations?”, https://fluke.com/en-in/learn/blog/renewable-energy/cat3-clamp-meter , opened 2026-10-28.