Klein Tools MM400 Auto-Ranging Digital Multimeter

The Klein Tools MM400 is an auto-ranging digital multimeter carrying a CAT III 600V rating independently verified by ETL (Intertek certificate #5000573) to UL 61010-1 and CSA C22.2 #61010-1 standards — the certification mark is visible on the back-of-meter label. It is an averaged-response meter (not True-RMS), which means it delivers accurate readings on clean DC circuits and standard sine-wave AC but will read 10–15% low on PWM-controlled automotive signals found in modern motor controls, LED lighting drivers, and variable-frequency loads. The feature set covers AC/DC voltage up to 600V, AC/DC current up to 10A, resistance to 40 MΩ, continuity with audible beep, diode test, capacitance, frequency, duty-cycle, and temperature via a K-type thermocouple adapter (included, range 0°F to 1000°F / −18°C to 538°C). Backlit 4-digit LCD with MAX/MIN hold. Community reception at 4.6 stars across 4,162 Amazon ratings confirms accurate, reliable performance for basic automotive and residential electrical work.

A home mechanic checking battery voltage, fuses, relays, sensors, and simple 12V automotive circuits will find the MM400 accurate and reliable — and the ETL Listed CAT III 600V certification means it's genuinely safe to probe household AC circuits, not just rated for it on a label. The comprehensive feature set including temperature, capacitance, and duty-cycle handles most diagnostic tasks. Skip it if the work involves modern vehicles with heavy PWM systems — variable-speed blower motors, LED headlight drivers, inverter-controlled accessories, or any circuit with a non-sinusoidal waveform — because an averaged-response meter will give wrong readings on those signals. The Klein Tools MM600 or any True-RMS meter in the same price range serves that use case. Also skip it if NCV (non-contact voltage detection) is needed for live-wire identification — the MM400 does not include NCV.

The MM400 carries an ETL Listed mark (Intertek certificate #5000573), certifying conformance to UL Std 61010-1, 61010-2-030, and 61010-2-033, and certification to CSA Std C22.2 #61010-1, 61010-2-030, and 61010-2-033. ETL is an Intertek NRTL mark — equivalent to UL Listing or CSA certification in terms of independent laboratory accreditation. The CAT III 600V rating means the meter's input protection has been tested for the fault energy levels present in residential wiring and distribution-level circuits, not just automotive DC. Powered by a single 9V alkaline battery (included); alkaline batteries are accepted at Call2Recycle drop-off locations and most major retailers. The ABS housing carries a Proposition 65 warning. Drop-rated to 3.3 ft for jobsite durability.