Choose the symptom or technical question that matches your enquiry. These 50 articles explain what to record, how to interpret the evidence and when specialist assessment is appropriate. They do not replace vehicle-specific repair instructions.
All diagnostic collections · Vehicle and module finder
Sensor measurements and interpretation
- Sensor Ground and Chassis Ground Are Not Always the Same Test Point
- Sensor Supply Voltage Is Correct Until the Sensor Is Connected
- Two Temperature Sensors Disagree Before a Cold Start
- Pressure Sensor Reading Uses Absolute Pressure Rather Than Gauge Pressure
- A PWM Sensor Signal Cannot Be Judged from Average Voltage Alone
- Sensor Signal Stays Fixed While the Physical Condition Changes
- Replacement Sensor Has the Same Plug but a Different Range
- Sensor Connector Pins Look Clean but the Fault Is Intermittent
- Several Sensor Codes Share One Supply Circuit
- Sensor Live Data Updates Slowly Compared with the Physical Event
Starting requests and restart symptoms
- Starter Clicks Once but the Engine Does Not Turn
- Starter Turns Slowly Only When the Engine Is Hot
- Start Button Responds Only After Several Presses
- Automatic Car Starts in Neutral but Not in Park
- Manual Car Will Not Crank Unless the Clutch Is Pressed Unusually Far
- Engine Cranks Longer After an Overnight Park
- Engine Starts Normally After a Second Immediate Attempt
- Starter Keeps Running After the Engine Has Started
- Vehicle Does Not Wake When the Door Is Opened
- Start-Stop Restarts the Engine Harshly at Traffic Lights
Power distribution and protective circuits
- Battery Terminal Looks Tight but Starting Voltage Drops
- Main Fuse Link Fails Again After Replacement
- Relay Clicks but the Controlled Component Does Not Operate
- Electronic Fuse Output Shuts Down Instead of Blowing a Fuse
- Several Cabin Accessories Fail from One Power Outlet Circuit
- Auxiliary Battery Warning on a Vehicle with Two Batteries
- Battery Monitoring Sensor Was Disconnected During Accessory Installation
- Electrical Warnings Appear When the Rear Demister Is Selected
- Ground Strap Replacement Did Not Resolve Intermittent Warnings
- Electrical Faults Began After Welding on the Vehicle
Emissions controls and monitored conditions
- AdBlue Level Warning Remains After Refilling
- AdBlue Quality Warning After an Unknown Fluid Was Added
- SCR Efficiency Fault Does Not Automatically Condemn a NOx Sensor
- NOx Sensor Replacement Followed by a Communication Code
- DPF Regeneration Is Inhibited by Another Engine Fault
- DPF Pressure Reading Changes After Hose Replacement
- Exhaust Temperature Sensor Reports a Fixed Extreme Value
- EGR Command and Feedback Do Not Match
- Oxygen Sensor Heater Fault Appears Only on Cold Starts
- Emissions Warning Returns After a Software Update
Electrical test methods and their limits
- Current Clamp Direction Can Reverse a Diagnostic Graph
- Oscilloscope Time Scale Can Hide a Brief Electrical Dropout
- A Test Lamp Is Not Suitable for Every ECU Circuit
- Resistance Checks on a Powered Circuit Can Mislead Diagnosis
- Back-Probing Damage Can Create a New Connector Fault
- Battery Support Equipment During Programming Is Not Just a Charger
- A Known-Good Waveform Must Match the Test Conditions
- A Bench Power Supply Current Limit Can Prevent a Module from Starting
- Thermal Camera Hotspot Does Not Automatically Identify the Failed Component
- A Continuity Beep Can Exist Through an Unintended Circuit Path
Need help choosing the next step?
Describe your fault or explore the main repair library. Confirm scope and arrangements before booking or dispatch. Workshop assessments take place in Didcot; suitable removed-module enquiries are welcome from across the UK.