FAULT FINDING GUIDE

Solar Panels Not Working: Solar PV Fault Finding Guide

Your solar PV system has stopped generating, is underperforming, or your inverter is showing a fault code. This guide explains the most common causes — inverter faults, DC isolator failures, string faults, shading — and tells you when you need an MCS-certified installer.

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12 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

  • 1The most common cause of a solar PV system producing no output is an inverter fault — most inverters display fault codes and log events that make remote diagnosis straightforward via the manufacturer's monitoring app.
  • 2DC isolator failures are a known issue on installations completed between 2010 and 2016. Faulty isolators can cause arc faults and fires; any DC isolator that is warm to the touch, discoloured, or showing increased contact resistance must be replaced immediately.
  • 3String faults — caused by a failed panel, junction box failure, or broken string cable — cause partial output loss. The characteristic signature is one or more strings producing zero or reduced voltage while others remain normal.
  • 4Shading from trees, chimneys, satellite dishes, or neighbouring buildings can reduce output by 10 to 40 per cent. MCS monitoring data showing consistent output reduction at predictable times of day typically points to a shading issue rather than an electrical fault.
  • 5Any work on a solar PV electrical installation — including DC-side fault finding — must be carried out by an MCS-certified installer or a competent person with appropriate qualifications. The DC voltage on a string of panels can exceed 600 V and is present whenever there is daylight.
01 · Fault Finding Guide

Solar Inverter Faults

The inverter is the most complex component in a solar PV system and the most common source of faults. Modern string inverters monitor their own performance continuously and log fault events with timestamps, making remote diagnosis via the manufacturer's app or portal the first step in any fault investigation.

  • Grid fault / grid disconnection — the inverter cannot synchronise with the grid frequency or voltage and has disconnected for safety (anti-islanding protection under G98/G99). Often a transient event that self-clears when the grid stabilises. If the fault recurs frequently, check for local grid quality issues with the distribution network operator (DNO).
  • Isolation fault / insulation alarm — the inverter has detected that the DC insulation resistance between the PV string and earth has fallen below its minimum threshold (typically 1 MΩ). This can indicate damaged panel wiring, a faulty junction box, or water ingress into a connector. Do not reset the system — contact an MCS-certified installer immediately.
  • Over-temperature shutdown — the inverter ambient or heatsink temperature has exceeded its operating limit. Common in summer with inadequate ventilation around wall-mounted inverters. Ensure a minimum clearance of 200 mm around the inverter on all sides as specified by most manufacturers.
  • MPPT / string voltage fault — the DC input voltage from the string is outside the inverter's maximum power point tracking (MPPT) window. This can occur if a panel has failed open-circuit (increasing string voltage) or failed short-circuit (reducing string voltage).
  • Inverter end of life — string inverters typically have a service life of 10 to 15 years. Gradual efficiency reduction, increasing fan noise, capacitor swelling, or recurring faults on an older inverter may indicate that replacement is more cost-effective than repair.
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02 · Fault Finding Guide

DC Isolator Issues

DC isolators on solar PV installations — particularly those installed between approximately 2010 and 2016 — have been the subject of significant safety concerns. Certain isolator designs were found to suffer accelerated contact degradation, leading to increased contact resistance, overheating, and in some cases arc faults and fires.

  • Warning signs of a failing DC isolator — the isolator enclosure is warm or hot to the touch during normal operation, the enclosure is discoloured or shows scorch marks, there is a burning smell near the isolator, or the system output has declined without another apparent cause. Any of these signs requires immediate action — switch off the system via the AC isolator and contact an MCS-certified installer.
  • Replacement requirement — the MHCLG and MCS issued guidance following a number of DC isolator fires. If your installation has the original DC isolator from 2010 to 2016 and it has not been inspected or replaced, arrange an inspection with an MCS-certified installer. Many installers offer free or subsidised DC isolator replacement campaigns.
  • Contact resistance testing — a qualified engineer can measure contact resistance across the DC isolator contacts under load to identify degradation before it becomes a fire hazard. This is recommended as part of any periodic solar PV inspection.
03 · Fault Finding Guide

String Faults

A string fault affects one or more panels within a series string, causing partial or total loss of output from that string. Multi-string inverters and systems with microinverters are more tolerant of single-panel faults, but series string systems (the most common configuration on UK residential roofs) will lose the entire string's output if one panel fails to conduct.

  • Open-circuit panel failure — one panel in the string fails to conduct, breaking the series circuit. All panels in the string drop to zero output. The inverter will report a string voltage error. Diagnosis involves measuring open- circuit voltage across each panel to identify the failed unit.
  • Junction box failure — the junction box on the rear of each panel houses bypass diodes that allow current to route around shaded cells. Bypass diode failure can cause a panel to either block the string (open circuit) or short individual cells (reducing panel voltage). Junction box failures are identified by thermal imaging during irradiance.
  • MC4 connector failure — the MC4 connectors used to link panels in a string can develop high-resistance connections due to incorrect mating, corrosion, or physical damage. High-resistance connections generate heat and can cause localised cable damage and arc faults. Visual inspection and IR thermography can identify hot connectors.
  • String cable damage — DC string cables routed across the roof can be damaged by UV degradation, physical abrasion, bird or vermin attack, or foot traffic during maintenance visits. Insulation damage causes leakage current, reducing insulation resistance and triggering the inverter isolation alarm.
04 · Fault Finding Guide

Shading Problems

Shading is a major cause of underperformance in UK solar PV systems. Even partial shading of a single cell in a series string can reduce the output of the entire string due to the bypass diode architecture. Identifying shading as the cause of underperformance requires comparing monitoring data against irradiance data for the location.

  • New shading sources — trees that have grown since installation, new neighbouring buildings, repositioned TV aerials or satellite dishes, and dormer extensions are common causes of shading that was not present during the original system design. Compare output against previous years at the same time of year to identify new shading.
  • Self-shading — chimneys, roof features, and antenna masts on the same building can cast shadows at specific times of day. The shadow pattern is predictable — underperformance at a consistent time of day on sunny days suggests self-shading rather than a fault.
  • Soiling — bird droppings, leaves, moss, and dust accumulation shade individual cells and reduce output. Soiling is most prevalent in areas near woodland or with significant bird activity. Annual cleaning is recommended for most UK roof installations.
  • Mitigation options — power optimisers or microinverters fitted to each panel allow module-level MPPT and dramatically reduce the impact of partial shading. For systems with significant new shading, retrofitting optimisers may be more cost-effective than other interventions.

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05 · Fault Finding Guide

MCS Monitoring and Performance Tracking

MCS-certified installations are required to include a monitoring capability. Modern inverters provide cloud-connected monitoring that records generation data, fault events, and performance metrics. This data is the most powerful diagnostic tool available for fault finding without accessing the roof.

  • Performance ratio — the ratio of actual output to the theoretical maximum given the irradiance. A healthy UK residential system should maintain a performance ratio of 75 to 85 per cent. A sustained drop below 70 per cent (after accounting for temperature effects) warrants investigation.
  • Fault event log — inverter monitoring platforms log every fault event with a timestamp and error code. Reviewing the fault log shows whether a fault is a one-off transient event or a recurring pattern that indicates a developing hardware problem.
  • String-level monitoring — two-string and multi-MPPT inverters report voltage and current for each string independently. A discrepancy between strings under equal irradiance immediately localises the fault to the lower-output string and eliminates the inverter as the source.
06 · Fault Finding Guide

When to Call an MCS-Certified Installer

Any electrical fault finding on the DC side of a solar PV installation requires a qualified person with specific PV training. The DC voltage present on a string of panels is potentially lethal and cannot be removed without covering the panels or waiting for darkness.

  • Call immediately — any isolation fault alarm on the inverter, any burning smell or visible scorch marks near the DC isolator or inverter, any warm or discoloured DC isolator enclosure, or any physical damage to roof-mounted cables or connectors. Switch off via the AC isolator and call an MCS installer.
  • Call as soon as convenient — output below 70 per cent of expected performance for more than one week (after ruling out shading and soiling), recurring inverter fault codes, string voltage errors, or an inverter more than 12 years old that has not been serviced.
  • Verify MCS certification — check the installer's MCS certificate number on the MCS installer database before commissioning work. MCS certification confirms the installer has the required training, tools, and insurance for PV electrical work.
07 · Fault Finding Guide

For Electricians: Solar PV Fault Finding and Certification

Solar PV fault finding and periodic inspection is an increasingly valuable specialist service. With over 1.3 million UK solar PV installations — many of them now 10 to 15 years old — there is growing demand for competent engineers who can diagnose faults, service aging systems, and issue compliant documentation.

Solar PV Installation Certificates

Use the Elec-Mate Solar PV certificate app to generate commissioning certificates on site. Covers system design, test results, DC isolator details, inverter data, and grid connection documentation.

EICR for PV Installations

When carrying out an EICR on a property with solar PV, the AC generation circuit and export meter connection must be included. Use the Elec-Mate EICR app to record generation circuit test results and note the PV system details in the schedule of particulars.

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Frequently Asked Questions About Solar Panel Faults

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