Find a guide that matches what your vehicle is doing. These guides explain what to note, what a test result can tell you and when to ask for help. A fault code or symptom alone does not prove that a module needs replacement.
If you are unsure, start with your symptoms or prepare a vehicle and module enquiry. Workshop assessments are in Didcot; contact AXION to confirm suitability and arrangements before travelling or sending a unit.
EV operation and drivetrain symptoms
- EV Shows READY but Will Not Select Drive
- EV Regenerative Braking Is Reduced After a Full Charge
- EV Power Is Limited at a Low Battery Percentage
- EV Makes a Clicking Sound Before READY Appears
- EV Warning Appears After Underbody Contact
- EV Reports an Electrical Fault Only After a Long Park
- EV Energy-Use Display Rises After a Tyre Change
- EV Range Estimate Changes After a Software Update
- EV Traction Warning Appears During Hard Acceleration
- EV Drive Noise Changes Between Acceleration and Coasting
Hybrid operation and engine transitions
- Plug-In Hybrid Uses Petrol with Charge Still Available
- Hybrid Engine Starts While the Car Is Stationary
- Hybrid Changes from Electric Drive to Petrol with a Jolt
- Plug-In Hybrid Electric Mode Becomes Unavailable After Refuelling
- Hybrid Battery Gauge Moves Quickly on a Steep Hill
- Hybrid Battery Cooling Intake Is Obstructed by Luggage
- Hybrid Fuel Consumption Rises on Repeated Short Trips
- Hybrid Will Not Enter READY After Interior Work
- Hybrid Engine Runs Roughly Only on Its First Start
- Hybrid Warning After Exhaust or Emissions-System Work
Braking, steering and chassis electronics
- ABS Warning Starts After a Wheel Bearing Replacement
- ABS Activates at Walking Speed on a Dry Road
- One Wheel-Speed Reading Drops Out on Corners
- Stability-Control Warning Appears After Wheel Alignment
- Electric Parking Brake Releases on One Side Only
- Parking-Brake Warning Remains After Rear Pads Were Changed
- Auto Hold Is Unavailable but the Parking Brake Works
- Electric Power Steering Is Heavy Only During Parking
- Steering Warning Appears After Driving Through Deep Water
- Suspension Height Warning After the Car Was Lifted
Seats, roofs and cabin controls
- Electric Seat Moves in One Direction but Not the Other
- Seat Memory Recalls the Wrong Position
- Ventilated Seat Fan Runs but the Seat Feels Warm
- Heated Steering Wheel Switches Off Before It Gets Warm
- Sunroof Stops Before Closing Fully
- Convertible Roof Stops Halfway Through Its Cycle
- Interior Ambient Lighting Has One Dark Section
- Horn Works from the Alarm but Not the Steering Wheel
- Rear Passenger Controls Fail but Front Controls Work
- Cabin Clock and Radio Presets Reset Overnight
ECU memory and circuit-board functions
- How ECU EEPROM emulation works without a separate memory chip
- Why ECU flash file size does not prove a complete backup
- What to check when two ECU reads differ
- Checking supply stability alongside an ECU memory error
- What an ECU Watchdog Does During a Controller Reset
- What an ECU power-management chip does
- How CAN controllers and transceivers work inside an ECU
- Why ECU circuit board coating needs careful handling
- Checking the connected load after an ECU output driver burns out
- ECU Component Markings Do Not Always Identify an Interchangeable Part
Connections, gateways and diagnostic access
- Diagnostic Socket Has Power but the Scanner Will Not Connect
- Scanner Reads Generic Engine Codes but Misses Manufacturer Functions
- Vehicle Network Fault Appears When a Trailer Is Connected
- Dashcam Installation Followed by Vehicle Communication Faults
- Network Faults Begin After a Windscreen Leak
- Connector Corrosion Without Visible Water Inside the Module
- Harness Repair Splice and an Intermittent Communication Fault
- Module Responds to Identification but Refuses Live Data
- Replacement Gateway Installed but Some Modules Are Still Missing
- Network Fault Appears Only with the Engine Running
Engine controls and fuel-system symptoms
- Engine Starts Then Stalls When Electrical Loads Are Switched On
- Engine Misfires Only When the Air Conditioning Is Switched On
- Fuel Gauge Reads Empty Immediately After Refuelling
- Engine Hesitates After an Aftermarket Intake Was Fitted
- Engine Temperature Reading Jumps When the Heater Is Used
- Diesel Preheating Warning Appears After Starting
- Engine Cooling Pump Commanded On but Circulation Is Uncertain
- Throttle Warning Appears After the Throttle Body Was Cleaned
- Engine ECU Fault Reported After Reversed Jump-Lead Connection
- Engine Fault Appears After Pressure Washing the Engine Bay
Transmission faults and repair handovers
- Gearbox Fault Appears Immediately After Battery Replacement
- Gearbox Warning Appears After a Fluid Service
- Gearbox Selects Neutral Unexpectedly at a Stop
- Gearbox Changes Normally in Manual Mode but Poorly in Automatic Mode
- Steering-Wheel Gear Paddles Stop Working
- Gearbox Temperature Warning While Towing
- Gearbox Fault Returns Only After the Vehicle Is Parked Hot
- TCU Repair Completed but the Gearbox Still Needs Vehicle Setup
- Used Mechatronic Unit Has the Right Connector but Different Identification
- Gearbox Fault Report Contains Engine Torque Messages
Keys, access and start authorisation
- Car Starts with One Key but Not the Spare
- Key Remote Works Only Very Close to the Vehicle
- Key Not Detected Warning Appears with the Key Inside
- Steering Lock Warning and No Ignition Response
- Replacement Key Unlocks the Car but Will Not Start It
- Car Starts but the Keyless Lock Button Does Not Respond
- Boot Opens Unexpectedly When the Key Is Carried
- Door Locks Cycle Repeatedly While Driving
- Immobiliser Warning Appears After an ECU Was Returned from Repair
- What to prepare when all your car keys are lost
Interpreting diagnostic reports and arranging repair
- What the extra characters in a fault code mean
- A Fault Is Present but No Code Is Stored
- A Code Clears with Ignition On but Returns When the Engine Starts
- What a fault frequency counter can tell you
- Diagnostic Report Uses the Wrong Vehicle Description
- Vehicle Works Normally When the Workshop Checks It
- What to check on the vehicle when a bench test finds no fault
- Sending a Module with Damaged Connector Pins
- What an ECU assessment and repair quote cover
- Returning a Repaired Module to a Different Garage
More technical guides
For the previous collection, browse vehicle electrical fault guides. Visit the automotive electronics repair library for fault-code explainers, module topics and other diagnostic resources. Use these guides to prepare your questions. Repairs still need the correct instructions for your vehicle and a qualified technician.