Warning What does this mean?

Your system is down or running degraded, but this is not an immediate safety danger. Work through the safe steps below.

Growatt Inverter Error 121: Meaning, Causes & Safe Fix

Error 121 means the inverter's master processor cannot receive data from the slave processor — the mirror image of Error 101. It is an internal communication fault that shuts the inverter down.

What should I do first?

Two quick questions — we'll point you to the right starting point.

Is this code still showing on your inverter's display?

What this code means

Your Growatt inverter runs two processors that must constantly talk to each other — a redundancy safety design. Error 121 means the master processor has stopped receiving data from the slave processor, so the inverter shuts down rather than operate without its safety cross-checks.

What is this downtime costing you?

Rough estimate while your system isn't producing. Adjust the numbers to match your setup.

System size (kW) Peak sun hours / day Electricity rate ($/kWh)

–per day

–per week

–per 30 days

≈ – kWh of lost production per day.

Estimate only — assumes ~80% real-world output vs. nameplate. Actual losses depend on weather, shading, panel direction, and your tariff.

Common causes

Internal communication bus fault
Control board failure
Firmware issue
Electrical interference or a loose internal connection

Safe steps to try

Work top to bottom, ticking off each step as you complete it. 0 of 5 done.

Note the exact error code on the display.
Shut the inverter down fully: turn off the DC switch and wait until the display goes completely dark.
Turn the DC switch back on and check whether the error returns.
If it clears, monitor for recurrence over the next day.
Check the Growatt monitoring app for any additional status information.

Is your inverter still under warranty?

Before paying for a repair, check — your installer may fix this free.

Brand Year installed

Based on typical standard warranty terms — always confirm with your warranty card or installer paperwork, as terms vary by model, region, and purchase date.

When to call an electrician

Call your installer or Growatt service if Error 121 persists after a full shutdown and restart — the control board may need replacement, which is an installer/service job.

Frequently asked questions

What is the difference between Error 121 and Error 101?

Both are processor communication faults: Error 121 means the master processor cannot hear the slave; Error 101 means the slave cannot hear the master. The remedy path is the same.

Can a firmware update fix Error 121?

Growatt's manual lists a firmware update as one possible remedy, but firmware work should be done by your installer — do not attempt it yourself.

Is it safe to keep restarting?

One or two orderly restarts are fine. Repeated power-cycling will not fix a hardware communication fault, so call for service instead.

Need a professional?

This fault may need professional diagnosis. Contact a qualified solar electrician in your area.

Sources: meaning and troubleshooting verified against the manufacturer's documentation — Troubleshooting for Growatt TL&MTL (Ver1.2) 2013-7-15, Growatt New Energy ('the two CPU units have something wrong with the communication to each other'); direction of fault ('Master processor can't receive data from Slave processor') per Growatt 1-3K-US User Manual