Diagnostic guide · SolarEdge P404

SolarEdge optimiser communication failure — what P404 means and how to fix it

SolarEdge Go is showing a P404 fault code — optimiser pairing lost. The inverter has stopped communicating with one or more optimisers over the DC power line. This guide explains what's causing it, whether a reset will fix it, and when you need an engineer on site.

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Optimisers dropping off?

Tell us how many optimisers show P404, whether it appeared after a firmware update or gradually, and any SetApp screenshots. We confirm whether it's a wiring fault or failed units before any site visit.

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SolarEdge hub All problems No production Red light Arc fault HD-Wave failure Optimiser N/A Comms failure Fault codes →

P404 is the fault code — N/A is the monitoring status. They're related but not the same.

P404 ("optimiser pairing lost") is the active fault code reported by SolarEdge Go when the inverter has lost its communication pairing with an optimiser over the DC power line. N/A is the label the monitoring portal displays when no data is arriving from that panel. In most cases, a P404 fault produces an N/A status — but the code gives more diagnostic detail about timing and cause. If you're seeing N/A in the monitoring portal but haven't checked SolarEdge Go, do that first — the Go app may show a P404 code that significantly narrows the investigation.

Not sure which page you need? See the optimiser showing N/A guide for the monitoring portal view and pattern-based diagnosis.

Diagnosis steps

Step by step

How to diagnose SolarEdge P404 communication failure

The key question is whether P404 is a pairing/firmware issue (often resolvable with a reset) or a physical fault (needs an engineer). These steps determine which it is.

Open SolarEdge Go and confirm P404 — note the count and pattern

Connect to your inverter via SolarEdge Go on Bluetooth. Stand within 3–4 metres of the inverter. Navigate to Alerts or the Device Status section. Confirm you're seeing P404 — "Optimiser pairing lost" — and not a different code. Take note of:

✓

How many optimisers are reporting P404

✓

Whether SolarEdge Go shows serial numbers or string positions for the affected units

✓

When the fault first appeared — check the event log timestamp

✓

Whether the P404 appeared suddenly (immediately post-firmware update, after a power event) or gradually

Cross-check with the monitoring portal

Log into monitoring.solaredge.com and check the layout view. Optimisers with P404 will typically show as N/A (greyed out). Note whether they are in a consecutive run or scattered — this distinction is covered in step 3.

Check whether a firmware update ran in the past 48–72 hours

In the monitoring portal, go to the system's inverter details page and check the event log for any firmware update activity. SolarEdge pushes OTA updates to inverters automatically — sometimes without any notification to the system owner. A firmware update is one of the most common triggers for P404 in otherwise healthy systems, because the update process can break or reset the inverter's pairing table with its optimisers.

P404 appeared after a firmware update

This is a pairing re-establishment issue. Try the soft reset in Step 3 — most post-firmware P404 faults clear within one or two resets. If they don't, a manual re-pairing via SolarEdge Go by an engineer is the likely fix.

P404 appeared with no firmware update

More likely a physical cause — wiring break, failing connector, or failed optimiser. Proceed through the steps, but expect an engineer visit is likely needed.

Read the pattern — consecutive or scattered?

The pattern of P404 faults in the monitoring portal layout is one of the most important diagnostic signals. It tells you whether to expect a single root cause or multiple independent failures:

Single optimiser P404

Usually a failed optimiser unit or a single faulty connector at that unit. The rest of the string is unaffected. Check warranty — 25-year cover applies.

Consecutive run of P404

PLC chain broken at the first affected unit. Single root cause — look for a connector or cable fault at or just before that panel. Not multiple failures.

All optimisers P404

Inverter-side communication issue. Often firmware or a DC bus fault. Check whether the inverter display is lit and whether any string data shows in the portal.

Scattered P404 (non-consecutive, across different strings) is rarer and may indicate multiple ageing optimisers approaching end of life, particularly in older pre-2016 installations.

Attempt a soft reset — AC isolator off for 90 seconds, then on

Before calling an engineer, try one soft reset. The 90-second wait (longer than a standard reset) allows the optimisers' PLC pairing state to fully clear before the inverter restarts and attempts re-pairing. At the consumer unit, switch the solar PV circuit breaker OFF. Wait 90 seconds. Switch back ON. Allow 15–20 minutes for the system to restart and the monitoring portal to refresh.

P404 clears after reset

The fault was a pairing glitch or post-firmware disruption. Monitor over the next 3–5 days. If P404 does not return, no further action needed. If it returns intermittently, book an engineer — intermittent P404 suggests a marginal connector or cable developing a fault.

P404 persists after reset

Physical fault confirmed. There is a break in the DC power line communication chain — a wiring fault, connector fault, or failed optimiser. An engineer visit is required. Do not continue resetting.

Only attempt a reset if you have not seen P700 or P701 (arc fault) codes. Arc fault events must not be reset — see the arc fault guide if those codes are present.

Contact a qualified engineer for re-pairing or physical repair

If the P404 persists after a reset or keeps coming back, book an engineer. Have ready:

✓

The P404 fault code confirmed in SolarEdge Go

✓

How many optimisers are affected and whether they are in a run or scattered

✓

Whether a firmware update preceded the fault

✓

Whether a soft reset was attempted and the outcome

✓

The inverter model and approximate install date

✓

A screenshot of the monitoring portal layout view showing the N/A pattern

The engineer will connect via SolarEdge Go and attempt a re-pairing sequence first. If that does not restore communication, they will proceed to physical DC string testing and inspection to find the underlying cause.

Root causes

Causes

What causes SolarEdge P404 communication failure?

P404 has three distinct cause categories — each with a different resolution path. Knowing which you're dealing with determines whether this is a same-day reset fix or a site visit.

Firmware update disruption — most common cause of sudden P404 on a healthy system

SolarEdge automatically pushes OTA firmware updates to inverters, typically overnight. The update process can disrupt the inverter's pairing table — the internal record of which optimisers are registered to the system and on which string positions. When the inverter restarts after the update, some or all optimisers may fail to re-pair automatically, producing P404.

This is not a hardware fault — the optimisers themselves are undamaged. The inverter simply needs to re-establish the PLC pairing with each unit. A soft reset (90-second power cycle) is usually sufficient. In cases where the pairing table has been significantly disrupted, an engineer may need to connect via SolarEdge Go and initiate a fresh pairing scan — a process that takes 10–20 minutes on site but does not require any physical work on the roof.

How to tell if this is the cause: P404 appeared overnight or early morning with no prior warning. The monitoring portal shows the fault starting at a specific time that correlates with a low-traffic period (typically 2–5 AM). All or many optimisers went to N/A simultaneously. The system was working normally the previous day.

DC power line communication break — wiring fault or failed connector

SolarEdge optimisers use power line communication (PLC) — they superimpose a high-frequency data signal onto the DC power cable to relay information back to the inverter. PLC is reliable when the DC circuit is intact and the cable quality is good. But any fault that increases resistance or breaks continuity in the DC line also breaks the PLC signal:

Degraded MC4 connector: increased contact resistance from corrosion or poor mating attenuates the PLC signal enough to drop communication, even if the power circuit still passes current. This often presents as intermittent P404 before it becomes permanent.

Cable damage: a partial insulation fault or constriction in the DC cable increases impedance, degrading the PLC signal. Rodent damage in the roof space is a common cause — partially stripped cable may pass power but attenuate the communication signal.

Open-circuit fault: a fully broken cable or disconnected connector stops both power and PLC communication entirely. All optimisers downstream of the break appear as P404.

Physical PLC breaks do not clear on reset — the fault is in the DC wiring and persists until repaired. The pattern of P404 (consecutive run starting at a specific panel) points the engineer to the right section of string.

Failed optimiser unit — hardware failure stopping both power and communication

A power optimiser that has failed completely stops functioning as both a power converter and a PLC relay. Depending on how it fails, it may either break the PLC chain (causing all downstream units to also show P404) or simply drop off the chain without affecting others. Either way, the monitoring portal shows the failed unit as N/A and SolarEdge Go reports P404 for it.

Hardware failure of an individual optimiser is more common in older installations (pre-2016) and in early S250/S270 series units. SolarEdge's 25-year warranty covers the replacement unit — but confirming the fault requires an engineer with SolarEdge Go to test the unit in-situ and document the failure before SolarEdge will process the warranty claim.

How to tell if this is the cause: Single P404 on one non-consecutive panel that doesn't clear on reset and has been present for more than a week. Or a run of P404s that starts at a specific optimiser position and persists after a pairing reset — suggesting the first unit in the run is the failed one breaking the chain.

On-site investigation

What to expect

What the engineer will do on site for P404

The engineer's approach for P404 differs depending on whether the cause is likely a pairing/firmware issue or a physical fault. SolarEdge Go allows a significant portion of the diagnosis to happen before any roof access.

SolarEdge Go Bluetooth session — P404 fault log and pairing test

The engineer connects to the inverter via SolarEdge Go and reviews the full fault log. They check when the P404 first appeared, which optimisers are affected, and what firmware version the inverter is running. SolarEdge Go's commissioning mode allows the engineer to initiate a fresh pairing scan — the inverter broadcasts a pairing request and all functional optimisers within range respond. This test immediately distinguishes a pairing issue (optimisers respond to the scan) from a physical fault (optimisers do not respond at all).

Re-pairing sequence — if the pairing test confirms a pairing issue

If the SolarEdge Go pairing scan finds the optimisers but they are not registered to the inverter's pairing table, the engineer will initiate a fresh pairing and commissioning sequence. Each optimiser is re-registered using its serial number. This process takes 15–30 minutes depending on the number of optimisers. Once complete, the system is restarted and the engineer confirms all previously P404 optimisers are now showing normal output data. No roof access is required for a pairing-only fix.

DC string continuity and voltage test — if the physical cause is suspected

If the pairing scan shows optimisers are not responding at all — meaning the PLC signal is not reaching them — the engineer will perform DC string continuity and voltage measurements at the inverter input. A break in the string will show as an open circuit or reduced DC voltage. The position and magnitude of the voltage drop narrows the physical location of the fault to a specific section of cable or a connector on the roof.

Roof inspection — connector test or optimiser replacement

With the fault location identified from step 3, the engineer accesses the roof and physically inspects the connectors and cable in that section. Faulty MC4 connectors are replaced. If the fault is in the cable, the damaged section is repaired or replaced. If an individual optimiser has failed, it is replaced — using the SolarEdge warranty claim process if within the 25-year warranty period. After any repair, the engineer restarts the system and confirms the repaired section is communicating and producing normally in SolarEdge Go before leaving the site.

How SolarEdge PLC works

How SolarEdge power line communication works — and why P404 happens

SolarEdge uses a communication method called power line communication (PLC) to send data between the optimisers and the inverter. Rather than separate signal wiring, the optimisers superimpose a high-frequency data signal directly onto the DC power cable. The inverter reads these signals from all optimisers simultaneously, giving it panel-level production data, fault codes, and control capability over each unit.

For PLC to work, there needs to be a complete, low-impedance DC circuit from the inverter through each optimiser and back. The optimisers communicate in a chain — each unit acting as a relay for those before it in the string. This architecture is why a break at a single point causes all subsequent optimisers to go silent: the relay chain is interrupted, and the data cannot reach the inverter past the break point.

The PLC pairing is established when the system is first commissioned. The inverter scans the string, discovers each optimiser by its unique serial number, and registers it in its pairing table. This table persists in the inverter's memory — but a firmware update that overwrites or corrupts the pairing table can require this process to be repeated. This is why a re-pairing sequence in SolarEdge Go can restore communication without any physical work on the roof when the cause is firmware-related.

P404 is the fault code that results when an optimiser that was previously in the pairing table stops communicating — regardless of whether the cause is a firmware disruption, a hardware failure, or a physical wiring fault. The code doesn't distinguish between these causes, which is why the diagnostic steps in this guide (pattern analysis, firmware check, soft reset, pairing test) are needed to identify which applies.

SolarEdge P404 communication failure — frequently asked questions

What does P404 mean on a SolarEdge system?

P404 is SolarEdge's fault code for optimiser pairing lost — the inverter has lost its PLC (power line communication) pairing with one or more optimisers. Unlike the N/A status shown in the monitoring portal, P404 is an active fault code reported by SolarEdge Go over Bluetooth. It can be caused by a temporary communication glitch (clears on a reset), a physical fault in the DC string wiring or connectors (persistent, needs investigation), or a firmware update that needs the pairing re-established. P404 is not a safety event — don't confuse it with P700 or P701 (arc fault).

What is the difference between P404 and optimiser N/A?

They describe the same condition from two different perspectives. N/A is the status the MySolarEdge monitoring portal shows when an optimiser stops reporting data — no data available from that unit. P404 is the active fault code SolarEdge Go shows over Bluetooth when the inverter has explicitly lost its communication pairing with that optimiser. In practice a P404 in SolarEdge Go usually corresponds to an N/A in the portal. The difference matters for diagnosis: SolarEdge Go's P404 gives more detail on timing, the specific affected units, and the re-pairing function, while the portal's layout view shows the pattern more clearly.

Can a SolarEdge firmware update cause P404?

Yes — it's one of the most common triggers for P404 on a system that was working fine. SolarEdge pushes OTA (over-the-air) firmware updates automatically, often overnight, and the update can disrupt the inverter-to-optimiser pairing, producing P404 for some or all optimisers right afterwards. A soft reset — AC isolator off for 90 seconds, then back on — clears most post-firmware P404 faults and the optimisers re-pair on their own. If P404 persists after two soft resets following an update, a qualified engineer should connect via SolarEdge Go to run a manual re-pairing sequence.

Why do multiple optimisers show P404 at the same time?

Multiple P404 faults in a consecutive run almost always share a single root cause rather than several independent failures. SolarEdge optimisers communicate over the DC power line in a chain, so if the chain breaks at one point — a failed connector, a cable fault, or a failed optimiser — every unit downstream of the break also loses communication and shows P404. The engineer traces the string from the first affected optimiser towards the panels to find the break. If P404 hits every optimiser across the whole system at once, the cause is more likely a firmware issue or an inverter-side communication problem.

Can P404 be fixed without a site visit?

Sometimes. P404 from a firmware update or a temporary glitch often clears after a soft reset (AC isolator off for 90 seconds, back on) with no on-site work — and we can talk you through that remotely for free. P404 from a physical fault, or one that won't hold after a reset, needs an engineer: SolarEdge Go re-pairs over Bluetooth within a few metres of the inverter, so re-pairing and any string repair are done on site. How the P404 behaves — did it appear after a firmware update? does it clear then come back? — tells us which category you're in before a visit is booked.

What does it cost to fix a SolarEdge P404 fault?

The remote diagnostic is free — send us your SolarEdge Go fault detail and a monitoring screenshot and we'll tell you, at no charge, which kind of P404 you've got. If it's a firmware or glitch P404, we'll talk you through the 90-second soft reset that usually clears it, also free, because you do it yourself. If the reset doesn't hold, re-pairing has to be done on site (SolarEdge Go pairs over Bluetooth, within a few metres of the inverter), and a physical fault — connector, cable, or a failed optimiser — also needs a visit; on-site work starts at £245. A failed optimiser inside its 25-year SolarEdge warranty is replaced with the unit supplied free by SolarEdge. You'll always have the cost before any work starts.

How quickly can a SolarEdge P404 be sorted?

A firmware or pairing P404 is often cleared the same day — once we've seen your SolarEdge Go detail we can usually say within about 30 minutes whether a soft reset will do it. If an engineer is needed, a SolarEdge Go pairing scan and re-pairing sequence on site takes roughly 30–45 minutes and usually needs no roof access. If a physical fault is confirmed (connector, cable, or a failed optimiser) the visit extends to about half a day, and a warranty optimiser replacement adds some admin time with SolarEdge though the swap itself is quick. Most P404s are resolved in a single visit.

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SolarEdge P404 — let's identify the cause and get communication restored.

We diagnose P404 faults using SolarEdge Go — starting with a pairing test and re-pairing attempt before any roof access. If the fault is physical, we trace the string and carry out the repair. Optimiser warranty claims managed with SolarEdge.

SolarEdge Go pairing scan and re-pairing sequence
Firmware-related P404 resolved without roof access where possible
DC string continuity and PLC signal testing
MC4 connector inspection and replacement
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