A fault code is a starting point, not a diagnosis

When a servo amplifier trips, the display gives you a code — not an answer. The temptation is to treat that code as the diagnosis itself: swap the drive, order the part, move on. But a large share of servo call-outs IAC's engineers hear about turn out to be wiring, encoder, mechanical binding, or parameter issues that a fault code merely points toward, not the drive amplifier failing outright.

This reference covers the three servo platforms IAC's customers ask about most: Yaskawa Sigma-7, Mitsubishi MR-J (J4/J5), and Siemens SINAMICS S120. For each, we've broken out the fault codes that come up most often on a shop floor, along with the first-action check before assuming the amplifier itself needs to be replaced.

This is a companion piece to IAC's PLC Fault Codes Decoded ↗ guide — together they cover the two systems most likely to trip a line to a stop with nothing more than a blinking code on a display.

Three brands, three numbering conventions

Before reading any fault code table, it helps to know how each manufacturer structures its codes — because the format itself tells you something about severity and scope.

Knowing the prefix convention alone tells you whether you're looking at something that stopped the axis (a true fault) or something flagging a condition worth investigating before it becomes one (an alarm/warning). Don't reach for a replacement part until you've confirmed which category you're in.

Yaskawa Sigma-7: the codes that show up most on a service call

The Sigma-7 series (SGD7S amplifiers) is one of the most widely deployed servo platforms in North American machine building. Most field calls trace back to a handful of recurring codes:

Note that Sigma-7 warning codes (the "A.9##" range) do not trip the drive and often clear on their own once the underlying condition — typically an overload margin or regeneration warning — resolves. Don't schedule a replacement based on a warning code alone.

Mitsubishi MR-J4 / MR-J5: reading the AL. code and its sub-code

Mitsubishi's MR-J series is common in packaging, semiconductor, and precision assembly equipment. Its two-part code — the main alarm number plus a decimal sub-code — is often skipped over in the field, but the sub-code is usually what actually narrows the cause.

Siemens SINAMICS S120: fault vs. alarm, and why the distinction matters

The S120 drive system is common in multi-axis machine tool and packaging applications, often paired with an S7-1500 CPU running motion control. Its five-digit codes are grouped by subsystem, and the F/A prefix tells you immediately whether the axis has stopped or is still running under a flagged condition.

The fault code that lies: encoder faults read as drive failure

Across all three platforms above, one pattern repeats more than any other: a fault code that names the drive or amplifier as the fault source actually traces back to the encoder cable — a loose connector, a pinched cable at a moving axis, or a failed connector shield letting in electrical noise. The amplifier reports what it sees (a lost or corrupted position signal), and what it sees looks identical whether the encoder itself failed or the signal simply never arrived cleanly.

The second most common misdiagnosis is treating an overload alarm as a defective amplifier when the real cause is a mechanical problem — a binding bearing, an undersized amplifier for a load that grew over time, or a motion profile with too aggressive an acceleration curve for the application. Swapping the amplifier in this case doesn't fix anything; the replacement unit will trip the same alarm on the same load.

None of this means the amplifier is never the problem — power section failures, blown IGBTs, and fan failures are real hardware faults. But running this checklist first, before a part is ordered, is the difference between a five-minute fix and an unnecessary replacement that doesn't resolve the actual fault.

When it is the amplifier, your line still can't wait

Once a first-action check confirms the fault sits with the amplifier, motor, or encoder itself, IAC stocks certified replacement servo drives and components across Yaskawa Sigma-7, Mitsubishi MR-J4/MR-J5, and Siemens SINAMICS S120 — plus the motors, encoder cables, and backup batteries that resolve the majority of the field calls covered above.

Warranty and verification

Every servo drive IAC ships carries a 2-year in-service warranty — twice the industry standard for refurbished industrial components. Amplifiers are bench-tested under load before shipping, and encoder feedback is verified at the connector level, not just powered on and inspected visually.

Same-day shipping

In-stock servo parts ordered before 4:00 PM Eastern ship same day. For urgent needs, call (877) 727-8757 during business hours — quote turnaround is typically under five minutes. You can also submit a part number via the quote form ↗ or email sales@iac.us.com.