Solar PV Fault Finding: Zero Output, Low Yield, Inverter Error Codes and Isolator Failures

Quick Answer: Solar PV faults fall into four main categories: zero output (inverter offline or DC fault), low yield (partial shading, soiling, degraded panel, inverter clipping), inverter errors (ISO fault, grid fault, over-temperature), and physical/mechanical faults (broken panel, corroded connectors, failed isolator). Diagnose with monitoring data first (when did output drop, by how much, which day/weather conditions?) before attending site. Most zero-output faults are either AC supply failure or a tripped RCBO, resolved in minutes; persistent low yield usually requires thermal imaging or string-level voltage testing.

Summary

A working solar PV system generates predictable amounts of electricity for 25+ years with minimal maintenance. When it stops working or underperforms, customers notice quickly — their electricity bill rises and their monitoring app shows red. Efficient fault finding minimises downtime and avoids unnecessary site visits for issues that can be resolved remotely.

This article covers the common fault categories, diagnostic approach, and how to resolve each. It assumes the installer has access to the monitoring platform and a basic test kit (multimeter, clamp meter, insulation tester).

Key Facts

• Monitoring data — the first tool for fault diagnosis; review the monitoring platform before attending site; compare current performance against historical data and expected generation for the time of year and weather conditions
• Zero output — the system produces nothing; causes: AC supply failure (RCBO tripped, inverter off), DC string fault, night/cloud (check monitoring timestamps)
• Low yield — the system produces less than expected; requires careful definition: how much less, over what period, compared to what reference?
• ISO fault (insulation fault) — inverter has detected reduced insulation resistance on DC circuit; stops generation as a safety measure; see solar pv earthing and bonding
• Grid fault — inverter has detected the AC grid voltage or frequency outside acceptable limits; usually transient and self-clears; persistent grid faults may indicate a distribution network issue or a wiring fault
• Over-temperature — inverter thermal protection has activated; usually in summer, poorly ventilated locations; generation stops until temperature falls
• RCBO trip — the generation circuit RCBO in the consumer unit has tripped; common causes: overload (inverter producing at full rated current), nuisance trip (earth leakage from inverter), AC supply fault
• Failed DC isolator — the DC isolator between array and inverter has developed a high-resistance contact (isolator failure is a known issue on older installations); causes reduced output or zero output
• Dirty/soiled panels — accumulated dirt, bird droppings, moss, or lichen can reduce output by 5–15%; cleaning typically restores performance
• Degraded panel — a panel that has developed a micro-crack or delamination; usually visible on thermal imaging as a hot spot; output reduced but rarely zero
• Corroded MC4 connector — DC connectors on the roof are exposed to weather; if not fully connected or if water has entered the connector, resistance increases and output drops; can cause heat and fire risk in severe cases
• PID (Potential-Induced Degradation) — a degradation mechanism in some panel types under high string voltage conditions; manifests as progressive low yield; more common in large commercial strings at high voltage; rare in UK domestic systems

Quick Reference Table: Fault Symptoms and First Checks

Detailed Guidance

Step 1: Remote Diagnosis from Monitoring Data

Before attending site, review the monitoring platform:

Zero output investigation:

• Check the timestamp of the last generation: was it last night (normal — inverter shuts down at night) or several days ago?
• Check the weather data for the location on the days of no output: complete cloud cover may explain briefly reduced output but not multi-day zero output
• Check the monitoring platform for error codes or alarms: most platforms (Fronius Solar.web, SolarEdge monitoring, Enphase Enlighten) show historical error events

Low yield investigation:

• Download monthly/annual production data
• Compare against previous same months (accounting for weather differences)
• Compare against the PVGIS yield estimate for the location (see solar pv system sizing)
• If the yield was normal for 2 years then dropped: something has changed (new shading, panel failure, inverter degradation)
• If yield has been low since installation: original commissioning issue (wrong string design, shading not identified in survey)

String-level analysis (SolarEdge/Enphase): If the system has panel-level or string-level monitoring, identify which panel or string is underperforming. This dramatically narrows the site investigation.

RCBO Trip and AC Supply Fault

Symptom: Inverter is completely dead; monitoring shows offline.

First check:

• Ask the customer: is the inverter LED on or off?
• Check the consumer unit: is the generation circuit RCBO tripped?

If RCBO tripped: Reset the RCBO. If it holds: the trip was a transient (power surge, brief earth fault); monitor and see if it recurs. If it trips again immediately:

• Test the AC cable insulation resistance (500V DC megger, L-E, N-E): if low, there is cable damage
• Disconnect the inverter from the AC and retest: if the cable tests clear with the inverter disconnected, the fault is in the inverter itself (AC input fault; contact manufacturer)
• Check for excessive earth leakage from the inverter (inverter DC-to-ground leakage can trip sensitive AC RCDs — see CT clamp issues section)

DC String Fault

Symptom: Inverter is powered on and shows AC grid connection, but DC generation is zero or very low.

On-site investigation:

1. Check the DC isolator: switch to off; open the DC connection compartment; confirm the conductors are intact and insulation is undamaged
2. Measure Voc at the inverter DC input (with DC isolator open and strings disconnected from inverter): each string should show a positive voltage approximately equal to the number of panels × Voc_per_panel
   • Zero Voc: open circuit somewhere in the string (loose MC4, broken panel junction box, severed cable)
   • Partial Voc (e.g., 300V where 400V expected): one or more panels missing from the string; count and check
3. If Voc is present but MPPT is not tracking: check inverter MPPT settings; check for a failed MPPT input on the inverter (confirmed by swapping the string to another input if available)

Failed DC isolator: DC isolators (particularly older models pre-2015) are prone to internal arc damage, contact oxidation, and spring failure. A failed isolator may show normal Voc when disconnected from the inverter but drops voltage under load. Test by temporarily bypassing the isolator with a cable (for diagnostic purposes only, by a qualified electrician) — if yield recovers, the isolator is faulty. Replace with a DC-rated isolator conforming to BS EN 60947-3.

Inverter Error Codes

Every inverter manufacturer uses slightly different error codes, but the common categories are:

ISO fault (insulation fault): Reduced insulation resistance on the DC circuit. Steps:

1. Check DC cable routes on the roof for damage (walkover inspection)
2. Check MC4 connectors: any that are open, partially connected, or show moisture ingress
3. Test DC insulation resistance per string (500V DC megger, strings disconnected from inverter)
4. The string with the lowest insulation resistance is the likely fault; inspect that string's panels and connectors

Grid fault / AC grid error: Inverter has detected grid voltage or frequency outside acceptable limits.

• If transient and self-clears: usually a local distribution network event; not an installation fault
• If persistent: check AC voltage at inverter terminals with a multimeter; if voltage is out of range (below 200V or above 253V), report to the DNO
• If voltage is normal but error persists: inverter protection settings may have been corrupted (firmware issue; contact manufacturer)

Over-temperature: Inverter has exceeded its thermal limit. Check:

• Is the inverter in direct sunlight? Inverters should be in shade or an enclosure with ventilation; direct sun on a south-facing wall can raise inverter enclosure temperature by 20–30°C above ambient
• Is the ventilation path (air inlet and outlet) clear? Cobwebs, dust, or an object placed in front of the inverter can block cooling
• Has the ambient temperature in the inverter location increased (e.g., new greenhouse nearby, building insulation changing air movement)?
• In the short term: improve ventilation or shading; in the long term: relocate the inverter if the thermal issue is structural

Low Yield: Soiling and Physical Inspection

Where remote analysis suggests low yield without a specific error code:

On-site visual inspection:

• Inspect panel surfaces: heavy bird droppings, dust, moss growth, or lichen?
• Inspect string cabling: any cables that have been displaced, damaged, or where the sheath has been breached by UV degradation or vermin?
• Inspect mounting: any panels that have shifted or slipped on the rails?

Cleaning: Panel soiling typically accounts for 1–5% yield loss annually in most UK locations. In heavily shaded or rural locations (birds, agricultural dust), soiling can be higher. Clean with deionised or soft water and a soft brush; do not use abrasive materials or high-pressure water jets on junction boxes.

Thermal imaging: Thermal camera inspection of the array (from the roof or drone) identifies:

• Hot spots: cells or panels with defects (micro-cracks, delamination, cell degradation) showing elevated temperature
• Cold spots: modules with shade-related bypass diode activation
• Abnormally hot strings: high-resistance connections (connector failure, junction box failure)

Thermal imaging is the most effective tool for identifying degraded panels without testing each one individually. It can be offered as a premium service for older installations or where yield issues cannot be resolved through other means.

Record-Keeping and Warranty Claims

For fault resolution that involves replacing panels or inverters under warranty:

• Document the fault with monitoring data extracts (showing the yield drop and date)
• Document site test results (Voc measurements, insulation resistance readings)
• Photograph evidence of the physical defect
• Contact the manufacturer's warranty team with this evidence

Most Tier 1 panel manufacturers and leading inverter manufacturers have UK warranty claim processes. Be prepared for a process that takes 2–6 weeks; keep the customer informed.

Frequently Asked Questions

The customer says output "seems low" but has no specific data. How do I investigate?

Request access to their inverter monitoring account and review the past 12 months of generation. Compare it to the system's expected annual yield (from the installation documentation) and to the same period last year if the system is more than a year old. If the current year's generation is within 10% of the expected figure, the system is likely performing normally; reassure the customer and set up a generation alert threshold in the monitoring platform.

My thermal camera shows a hot spot on one panel. Does the panel need to be replaced?

Not necessarily immediately. A hot spot indicates a localised defect (micro-crack, cell bypass diode activation, partial delamination). If the hot spot is associated with a string-level output reduction (confirmed from monitoring), the panel should be replaced under warranty. If the yield impact is minimal and the hot spot is not growing (compare thermal images 6 months apart), monitor and replace at the next scheduled maintenance visit.

The inverter is showing "PV arc fault" error. What does this mean?

A PV arc fault error indicates the inverter's arc fault detection circuit has detected a DC arc in the string. Arc faults in DC circuits (caused by a poor connection, damaged insulation, or failing connector) are a fire risk. Isolate the DC circuit immediately, inspect all MC4 connectors and junction boxes on the affected string, replace any damaged connectors, and retest insulation resistance before re-energising. Do not reset the error and ignore it.

Regulations & Standards

• BS EN 62446-1 — inspection and test documentation; baseline for comparing current vs original test values

• BS 7671:2018+A2:2022 Section 712 — periodic inspection requirements for solar PV

• MCS 001 — workmanship warranty obligations; fault response under warranty period

• IEC 62109 — safety of power converters; arc fault detection requirements

• IET Guidance Note 7 — Solar PV fault finding — diagnostic guidance

• Fronius inverter error codes — Fronius-specific fault code reference

• SolarEdge monitoring portal troubleshooting — panel-level fault identification

• solar pv earthing and bonding — ISO fault investigation and DC earthing

• solar pv commissioning handover — baseline test values for comparison

• string inverter vs microinverter — monitoring granularity and fault isolation capability

• solar pv system sizing — yield comparison baseline