MG Fault Codes, Explained in MG Context
✓ Technically reviewed by Daniel Wei Chen — Senior Chinese Automotive Diagnostics & Repair Specialist, Sydney, Australia · 20 August 2026.
A cheap OBD-II reader tells you what the ECU saw; it doesn’t tell you why — and generic code lists on the internet don’t know what MGs actually do. This section connects the standard code definitions (SAE J2012 / ISO 15031-6) to the documented MG-specific patterns from our problems research: which codes show up on which MGs, what an owner can safely check, and where the DIY line is.
How to read a code without fooling yourself
- The code is a symptom, not a diagnosis. P0562 means the ECU saw low voltage — it does not mean “replace the battery”.
- Generic vs manufacturer codes: P0xxx / P2xxx codes are standardised across all brands; P1xxx and many U-codes are manufacturer-specific and need MG-level documentation — we only publish what we can verify.
- Freeze-frame matters: note when the code set (cold start? highway? after a battery change?) — it’s often worth more than the code itself.
- Clearing a code doesn’t fix anything. It erases the evidence a workshop needs. Photograph first, clear later — if at all.
Codes documented so far
| Code | Standard meaning | MG context |
|---|---|---|
| P0562 | System voltage low | The 12V battery pattern documented across ZS EV, MG4 and HS |
| P0420 | Catalyst efficiency below threshold | The ageing petrol fleet’s most misdiagnosed code — and the one recall context matters for |
| P0300 | Random / multiple cylinder misfire | Service items first — but on early older-generation MG3s, the documented engine patterns raise the stakes |
| P0128 | Coolant below thermostat regulating temperature | The cheap code owners ignore — and shouldn’t, on the older MG3’s documented cooling pattern |
| P0171 | System too lean (bank 1) | The unmetered-air hunt — rubber age on the older fleet, boost pipes on the 1.5T |
| P0455 | EVAP large leak detected | Fuel cap first, always — plus a timing note for tank-recall MG3s |
| P0087 | Fuel rail pressure too low | The two-stage GDI fuel system on the 1.5T, and the old MG3’s documented pump weakness |
| P0A80 | Replace hybrid battery pack | A symptom report, not a repair order — and the warranty evidence you must not clear |
| U0100 | Lost communication with ECM | Real fault or 12V ghost? The triage that saves misdiagnosis |
Code definitions per SAE J2012 / ISO 15031-6 (generic OBD-II). MG-specific context from our documented research — each code page cites its own evidence. More codes are added as our workshop-case programme documents them: we publish verified patterns, not copied lists.
Tools: any ELM327-compatible reader with a reputable app reads generic codes on MGs. For hybrid and EV systems, dealer-level tools see far more than generic OBD — a “no codes found” from a cheap reader does not mean no fault is logged.
Disclaimer: Information on this page is general guidance only and does not replace the manufacturer’s official documentation or the judgement of a qualified technician. A fault code identifies a symptom the ECU observed — not, by itself, the failed part. Never attempt ECU reprogramming, high-voltage work or safety-system repairs yourself. Last verified: 20 August 2026 · Next re-check: November 2026.