AXION AUTOMOTIVE · TECHNICAL LIBRARY
Vehicle electronics diagnostic notes
Find focused explanations of sensor signals, ECU circuits, EV charging, battery monitoring and diagnostic response codes. Choose a subject below, then open the note that matches the result you are trying to understand.
These notes help you interpret evidence and prepare a useful enquiry. They do not replace the procedure for your exact vehicle. For a fault on your car, start with our services or the vehicle module finder.
Choose the system you are investigating
Open a subject to browse its 25 notes. Each note explains one signal, circuit behaviour or test result.
CAN signals and network fault finding
- CAN common-mode voltage when both signal wires move together
- CAN ringing after a harness joint or connector repair
- An extra CAN termination resistor can create a new fault
- Split CAN termination and the capacitor between the resistors
- Long diagnostic extension leads can disturb a CAN investigation
- CAN High and CAN Low swapped after connector replacement
- A CAN transceiver left in standby can look like a dead ECU
- CAN dominant-timeout protection and a transmission that stops
- CAN communication that disappears as a transceiver heats up
- CAN logic-supply compatibility when replacing a transceiver
- CAN FD connects at first but fails when the data phase starts
- CAN FD payload length and a misleading decoder display
- CAN FD sample-point settings in an intermittent diagnostic link
- A silent CAN logger and a lone controller on the bench
- CAN logger filters can hide the message being investigated
- CAN receive-buffer overrun can make a recording incomplete
- Standard and extended CAN identifiers in a diagnostic recording
- CAN CRC errors and an application checksum are different checks
- CAN stuff errors in a trace after wiring work
- CAN form errors and why the decoder setup matters
- CAN error counts do not identify the guilty ECU on their own
- CAN load percentage and a slow scan-tool response
- CAN resistance measured through a gateway can be misleading
- CAN voltage averaged by a multimeter can hide brief dropouts
- A CAN wake-up event is not proof the application started
LIN, Ethernet, FlexRay and diagnostic responses
- A quiet LIN wire between requests is not necessarily broken
- A LIN line held low and several devices unavailable
- LIN pull-up problems after a wiring or module repair
- LIN synchronisation faults when the decoder shows the wrong speed
- The long low pulse at the start of a LIN message
- LIN header present but the expected device does not answer
- Replacement LIN devices and the importance of the correct variant
- LIN wake activity without normal function afterwards
- Ethernet link established but DoIP diagnostics still unavailable
- DoIP connection failures after a laptop network change
- DoIP routing activation denied before a module operation
- DoIP requests for an encrypted connection
- Automotive Ethernet signal-quality readings during intermittent faults
- Automotive Ethernet master and slave configuration after module work
- Ethernet cable-distance estimates from a diagnostic test
- An Ethernet link that repeatedly connects and drops
- FlexRay start-up faults on an incomplete bench setup
- FlexRay channel A and channel B need separate fault evidence
- FlexRay synchronisation loss and intermittent chassis warnings
- UDS 0x11 service not supported during a diagnostic request
- UDS 0x12 subfunction not supported during module testing
- UDS 0x13 incorrect length or format in a tool report
- UDS 0x22 conditions not correct before an adaptation
- UDS 0x24 request sequence error during a guided procedure
- UDS 0x31 request out of range during coding or data access
Sensor signals and measurement faults
- SENT fast data and slow data can describe different sensor information
- SENT status information when the pressure value still looks reasonable
- SENT data nibbles combined in the wrong order
- SENT SPC trigger settings and a sensor that appears silent
- SENT decoding errors caused by an unsuitable signal threshold
- A SENT signal is not an ordinary duty-cycle measurement
- An inductive crank sensor with a weak signal only during cranking
- Hall and inductive crank sensors need different test methods
- A crank signal that changes after a sensor or flywheel replacement
- Active wheel-speed sensors can carry information through current changes
- Wheel-speed direction data can matter even when speed is displayed
- A magnetic wheel encoder damaged during bearing work
- Ratiometric sensor readings depend on the reference as well as the signal
- SENT calibration timing and a waveform that will not decode
- A sensor signal pinned near the supply is not always a failed sensor
- A sensor signal near zero can have several electrical causes
- A pressure sensor can share a housing with a temperature sensor
- An optional SENT pause pulse can be normal message timing
- ADC input settling can affect a module’s sensor reading
- Sensor readings that change when a test probe is connected
- SENT checksum errors need the correct message interpretation
- A digital sensor waveform can be clean but describe the wrong range
- Two sensor tracks disagreeing does not identify which track is wrong
- Temperature-related sensor faults need a recorded operating condition
- A waveform screenshot without its scale can mislead the diagnosis
ECU power supplies and component-level fault assessment
- A smart output switch can turn off to protect itself
- Repeated output pulses can be a protection retry sequence
- High-side current-sense readings need the correct interpretation
- An output open-load report with the component switched off
- Cold inrush current and an output that works after several attempts
- MOSFET on-resistance is only one part of replacement suitability
- MOSFET gate charge can matter after an output-stage repair
- MOSFET body-diode behaviour in a motor-control repair
- MOSFET safe operating area and a component that fails under load
- Repeated avalanche stress in an inductive output stage
- Reverse-polarity protection can block power reaching an ECU
- A shorted transient suppressor and a module supply fault
- Load-dump protection is different from an ordinary steady overvoltage check
- A PMIC fault state can hold a module inactive
- Window-watchdog faults can involve an update that is too early
- A reset line can remain active after supply voltage has recovered
- One internal ECU rail missing while another remains present
- Regulator dropout during an ECU start-up fault
- An output capacitor substitution can change regulator stability
- Thermal shutdown in a regulator can imitate an intermittent software fault
- Power-up sequencing matters during an ECU bench assessment
- Bench lead voltage drop can differ from the supply display
- Low resistance on an ECU rail does not automatically prove a short
- An ECU that works without its external load still needs circuit testing
- Repaired power electronics need a defined verification condition
Memory integrity and programming response codes
- EEPROM page boundaries and data written to an unexpected address
- An EEPROM temporarily not acknowledging after a write
- EEPROM addressing errors when the chip responds at an unexpected address
- SPI memory reads that change with the communication settings
- An EEPROM write-enable step is different from a completed write
- EEPROM endurance and data retention describe different limits
- Power interruption during memory writing needs a careful recovery assessment
- An all-FF EEPROM file is not automatically a valid blank memory
- Matching EEPROM reads do not prove the vehicle data is correct
- Secure boot can reject software that was transferred successfully
- UDS response 0x14 when the requested reply is too large
- UDS response 0x21 when a controller is busy
- UDS response 0x25 when a downstream module does not answer
- UDS response 0x26 when an existing fault prevents an operation
- UDS response 0x33 during authorised module programming
- UDS response 0x34 when authentication is required
- UDS response 0x35 does not mean the car key itself is faulty
- UDS response 0x36 after too many unsuccessful access attempts
- UDS response 0x37 while a required security delay is active
- UDS response 0x70 when a memory transfer is not accepted
- UDS response 0x71 when a data transfer is suspended
- UDS response 0x72 during an erase or programming operation
- UDS response 0x73 and the programming block sequence
- UDS response 0x7E when a subfunction is unavailable in the active session
- UDS response 0x7F needs the complete message to be interpreted
EV charging communication and battery measurement
- A battery cell-monitor open-wire flag is not a direct cell-capacity result
- Main and auxiliary battery-voltage measurements can disagree
- A battery monitor can report communication while its readings remain invalid
- A break in a battery-monitor daisy chain can hide several units
- Battery-monitor ring communication can retain partial access after a fault
- Several battery temperature readings moving together may share a cause
- A battery current offset can affect accumulated charge estimates
- Charge and discharge current signs can differ between diagnostic displays
- Kelvin sensing connections matter in a battery-current circuit
- Battery current-sensor range can matter during a brief high-load event
- Battery measurements taken at different times can create a false comparison
- An insulation-monitor self-test fault differs from a measured insulation fault
- Insulation measurement conditions can change when HV loads connect
- An insulation warning does not identify the battery pack as the only suspect
- EV charge-control signalling and energy conversion are separate stages
- Proximity and control-pilot information should not be treated as the same signal
- CCS power-line communication can fail before DC charging begins
- An EV charging session can fail at protocol negotiation
- Vehicle and charging-station logs should use a matching timestamp
- An on-board charger’s PFC stage and DC/DC stage do different jobs
- Galvanic isolation in an EV charger is part of its design
- Combined charger and DC/DC modules can have distinct fault paths
- An inverter gate-driver supply warning is not a motor-position diagnosis
- EV charging power and battery charging current are not the same measurement
- Battery-monitor replacement needs the correct system configuration
Diagnostic transport, timing and fault records
- An ISO-TP first frame without the rest of the diagnostic reply
- ISO-TP flow-control timeouts during a module data read
- ISO-TP block size can explain pauses in a diagnostic transfer
- ISO-TP separation time and a transfer that sends frames too quickly
- ISO-TP wait frames are different from a complete rejection
- ISO-TP overflow points to a receiving limit in the exchange
- An ISO-TP consecutive-frame sequence error needs the raw capture
- Unexpected ISO-TP consecutive frames after a diagnostic session restarts
- An unexpected ISO-TP flow-control message in a diagnostic log
- ISO-TP padding bytes are not part of the requested vehicle data
- ISO-TP addressing mode can shift where the payload begins
- Transmit and receive identifiers mixed up in a diagnostic setup
- CAN FD transport settings must agree with the diagnostic link
- An ISO-TP final frame can be shorter than earlier frames
- A new diagnostic message can interrupt an unfinished ISO-TP reception
- Diagnostic payload length and CAN frame length answer different questions
- Physical and functional diagnostic requests can produce different response sets
- UDS service identifiers are not the same as stored fault codes
- A positive UDS response confirms one requested operation
- A UDS data identifier needs the correct vehicle definition
- DTC status bits explain more than a simple present or stored label
- A DTC failure-type byte can change the meaning of a fault entry
- Diagnostic snapshot records need their own operating context
- Two scan tools can report different fault totals without different vehicle faults
- A diagnostic transport timeout is different from an ECU negative response
Motor, solenoid and actuator circuit assessment
- H-bridge motor voltage should be interpreted across the motor
- Motor braking and coasting can produce different stop waveforms
- A reversible motor working in one direction narrows the circuit checks
- Motor current regulation is different from overcurrent shutdown
- Motor start-up current and sustained stall current need different conclusions
- Brushed-motor current ripple can reveal rotation information
- Uneven motor current ripple needs comparison with the operating load
- Sensorless motor position estimates can depend on a valid current pattern
- A motor-driver fault latch can remain after the original event
- Motor-driver sleep or enable state can explain an inactive output
- Solenoid peak current and holding current serve different purposes
- Solenoid current can remain while the switched voltage changes
- Current-sense bandwidth can affect a solenoid test result
- A solenoid’s electrical response does not measure hydraulic pressure directly
- Transmission pressure measurements need the correct test point and condition
- A TCU output assessment should identify the exact controlled valve
- Mechatronic bench results need a clear statement of what was tested
- A turbo actuator position signal and actual boost are different evidence
- Contactless actuator feedback still depends on correct mechanical installation
- Pedal demand and throttle position do not always move one-to-one
- Throttle position feedback should be compared through its full permitted travel
- AC coupling can hide the DC level of a module supply
- Current-clamp zero drift can affect a small electrical-load reading
- Voltage and current recorded at different times can tell the wrong story
- A test report should separate commanded movement from achieved movement
Bring us the fault, even if you do not know the module
Drivers and garages can contact AXION with the vehicle details, symptoms and any saved fault report. No code or part number is needed to make an initial enquiry. Call 07440 134303.