Klein CL390 vs Fieldpiece SC440

Choose the Fieldpiece SC440 for heating service when AC motor-start diagnosis matters: it has documented 100-millisecond inrush capture alongside DC microamps for flame-sensor work. Choose the Klein CL390 when you need DC current through the jaw as well as AC; Klein documents Max/Min, but no dedicated inrush capture mode. Both cover lead-based DC microamps, capacitance and temperature, so those feature names alone do not settle the purchase.

Quick picks: choose the missing function

• AC motor-start troubleshooting: SC440. Buy for its specified inrush function, rather than assuming ordinary peak retention will capture the same event.
• Mixed AC/DC current work: CL390. Its jaw handles both current types; the SC440 jaw is AC only.
• Flame-sensor checks without motor-start capture: either. Compare the microamp specification, probe arrangement and included accessories before choosing.

For a wider shortlist, see our HVAC multimeter guide. This comparison concerns these exact models and their documented standard packages.

CL390 vs SC440: the differences that change the purchase

All figures below are manufacturer specifications, not independent measurements. Sources: Klein's CL390 page, its linked instruction manual, and Fieldpiece's SC420/SC440 manual.

Neither wins every row. The SC440 supplies the documented motor-start function; the CL390 supplies DC jaw current. A higher display count or capacitance ceiling does not compensate for the missing measurement you actually need.

Best for AC motor-start diagnosis: Fieldpiece SC440

The SC440 is our first choice for an AC-focused heating-service kit. Its manual documents inrush, a DC microamp function specifically for flame-rectifier tests, and a direct Type-K thermocouple socket. The dual display can show voltage and AC current simultaneously; it is useful when those are the paired readings you need.

The practical extras are a magnetic strap, a test-lead holder, backlight and automatic jaw light. Fieldpiece's US package listing also names silicone leads and short alligator extensions. Those documented accessories make it the more complete starting package for our AC-service pick.

The trade-offs: no DC current through the jaw, and the inrush mode has minimum trigger inputs explained below. Its temperature socket also requires removing the electrical leads and moving the TEMP switch; it does not promise temperature alongside an electrical reading.

Check the SC440 package at Fieldpiece.

Best for AC and DC clamp current: Klein Tools CL390

The CL390 makes more sense when your purchase must include DC jaw current. Klein specifies AC and DC clamp ranges, plus a separate lead-based microamp function, temperature, capacitance and Max/Min recording. Its reverse-contrast display and side lead holder are other reasons to consider it if the required functions already fit.

There is a useful limitation behind the large current number: 0.3 A is Klein's minimum measurable jaw current, despite finer display steps on its lower range. That specification is separate from the microamp function. Do not buy a large-current jaw expecting it to replace a lead-based flame-sensor measurement.

A magnetic hanger is optional. More consequentially, Klein's manual says not to use DC clamp current with a magnetic hanger attached, because the magnet can cause inaccurate measurements. Account for that restriction if hanging the meter is part of your normal workflow.

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Buying guide: separate flame current from motor current

DC microamps: both use leads

For flame-rectifier service, compare the lead-based range, not the jaw rating. The CL390 specifies 200 µA with 0.1 µA resolution and ±(1.0% + 5 digits). The SC440 specifies 500 µA with 0.1 µA resolution and ±(1.0% + 2 counts), plus 1 V burden. These are accuracy specifications with a percentage term and a least-significant-count term; resolution alone is not accuracy.

Fieldpiece explicitly directs the reader to compare a flame-sensor reading with the heating equipment manufacturer's specification. Use that equipment documentation to judge the result, rather than a universal pass/fail microamp value. Follow the instrument and equipment manuals for the measurement procedure.

Inrush: a defined capture mode beats an assumption

Fieldpiece specifies a 100-millisecond measurement period, with minimum input above 2 A on the 50 A range or above 20 A on the 400 A range. That makes the SC440 our pick when the motor-start task falls within those documented conditions. Do not assume that selecting the larger range improves capture of a smaller starting event.

Klein describes Max/Min as retaining maximum, minimum and their difference while continuing to sample. It does not give the CL390 an inrush capture specification. A retained maximum may be useful, but it is not evidence of equivalent starting-current capture.

• Yes: Choose SC440 when documented AC inrush capture is required.
• Yes: Choose CL390 when DC current through the jaw is required.
• Partly: Both provide DC microamps through leads; check the equipment's required signal.

Temperature: buy the probe arrangement, not the input ceiling

Each meter accepts a Type-K thermocouple, but the connections and supplied-probe limits differ. The CL390 manual, SC440 manual and ATB1 specifications establish these separate limits:

The usable setup must stay within both meter and probe limits. The CL390's higher input ceiling does not give its included probe that range. Likewise, the SC440 input ceiling does not upgrade the ATB1. Fieldpiece's ATB1 manual describes using the supplied Velcro strap to secure the probe to a pipe, so a separate pipe clamp is not automatically necessary.

For repeated clip-on pipe measurements with the SC440, our optional accessory is the Fieldpiece TC24. Its own range is −50 to 257°F / −46 to 125°C, it fits 3/8″ to 1 3/8″ pipe diameters, and it uses a Type-K male mini plug. That is a lower upper limit than the included ATB1, despite the convenient clamp format; also respect the SC440's lower input boundary. Do not assume the mini plug directly fits the CL390's electrical jacks.

TC24 documentation caveat: its page lists ±0.5°C after calibration and settling under 30 seconds; the linked manual's specification table lists ±0.6°C and 30 seconds typical. Treat those details as unresolved.

Compare complete kits and the exact model label

The makers document these standard contents:

Sources: Klein package contents and Fieldpiece package contents. Compare a bundle against this inventory before paying for duplicate accessories. Check that an advertised pipe probe is included in that particular bundle, rather than assuming it comes with the meter.

SC420 is a different model. Although Fieldpiece shares a manual between SC420 and SC440, that manual labels True RMS, inrush, backlight, jaw light and the magnet as SC440-only features. Match the meter label and package inventory; do not transfer every feature in a shared manual to its other model.

Klein documentation conflict: the CL390 product page says auto power-off after 10 minutes, while its linked manual says approximately 5 minutes. Confirm the supplied manual revision; we would not make either delay a purchase reason.

1. 1

   Match the current path

   Decide whether you need lead-based microamps, AC motor-start capture or DC jaw current.

2. 2

   Match the temperature setup

   Check the connector, probe limit and whether simultaneous temperature readings are required.

3. 3

   Check the package

   Compare included leads, clips, probe, case and hanging accessories before adding extras.

How we choose

We compared official product pages, the currently linked manuals, model-specific feature notes and accessory documentation, checked November 3, 2026. Recommendations reflect those documents; we have not physically evaluated these meters. We distinguish supplied-probe limits from meter ranges and leave the auto power-off conflict visible. Our selection method explains the research process.

Both instruments are marked CAT III 600V. Follow their manuals, inspect leads and use the required protective caps; an accessory does not upgrade the instrument's rating. Klein warns that no NCV indication can still mean voltage is present. Leave panel and service-equipment work to a licensed electrician.

For the broader instrument choice, read clamp meter vs multimeter. Keep a complementary handheld or thermometer when your job requires measurements that the chosen clamp cannot provide together. If your comparison is within Klein's family, use CL390 vs CL710.

Verdict

Buy the SC440 for an AC-focused heating-service kit when its documented inrush capture is the missing function. Buy the CL390 for AC/DC jaw current when Max/Min meets your needs. Both support lead-based DC microamps and temperature; choose the ranges, probe connection and complete package that match your work, and add the TC24 only when its pipe fit and separate temperature limit suit the job.