Symptom selector (fast filter)
Not sure which flow you need? Search by symptom, code or system and jump straight to the closest decision tree.
Open symptom selector →Fault Trees & Decision Flows
Pick your symptom, then follow a simple decision flow. Each step tells you what to check, what result means, and what to do next.
How to use these
- Start with the symptom (not the code). Codes are often downstream symptoms.
- Do the quick, free checks first (battery voltage, air leaks, connectors, fuses).
- Confirm with a test before replacing parts. Use the linked workshop guides if you’re unsure.
Tip: keep a notepad. Write down what you measured and what changed.
Cranks but won’t start
A structured flow for petrol & diesel: battery, immobiliser, spark, fuel pressure, injector pulse, and basic sensor sanity checks.
Open fault tree →Rough idle / misfire
Distinguish ignition, air leaks, fuel trim behaviour, MAF/MAP plausibility, and misfire counters.
Open fault tree →Overheating / cooling fan issues
Fan command logic, coolant temp plausibility, thermostat behaviour, and pressure/circulation checks.
Open fault tree →Battery/charging warning or low voltage
Voltage drop workflow, alternator output, belt drive, smart charging behaviour and parasitic drain basics.
Open fault tree →No crank / clicks
Battery terminals, grounds, starter relay/solenoid control, P/N or clutch switch, and immobiliser lockouts.
Open fault tree →Stalling / cuts out
Intermittent power/ground, crank sensor dropouts, fuel supply issues, throttle control and vacuum leaks.
Open fault tree →Boost loss / underboost
Pressure leaks, vacuum supply, wastegate/VNT control, MAP/MAF plausibility and actuator tests.
Open fault tree →Diesel limp mode (DPF/EGR/boost)
DPF soot/load and diff pressure sanity, EGR behaviour, boost control, and safe regen decision points.
Open fault tree →ABS / traction warnings
Wheel speed sensor patterns, tone rings/bearings, wiring, low voltage, and U-code communication basics.
Open fault tree →Hot / cold intermittent no-start
Heat soak, pressure bleed-down, crank signal loss, sensor plausibility and the quickest confirmations.
Open fault tree →Petrol no power / limp mode
Boost and air leaks, ETC/throttle faults, fuel trims, low fuel pressure, and knock/overheat protections.
Open fault tree →DPF regen decision flow
When a drive regen is safe, when a forced regen might help, and when you must stop and fix the cause.
Open fault tree →A/C not cold
Command, compressor/clutch, fans, pressures, leak clues, sensors, and blend doors in a simple flow.
Open fault tree →Brake pedal vibration / pulsation
Separate disc runout/pad transfer from ABS activation and suspension/wheel bearing issues.
Open fault tree →P0171 / P0172 fuel trim (lean or rich)
Use fuel trims properly: vacuum leaks, PCV, MAF plausibility, fuel pressure/injectors, plus O2 sensor sanity checks.
Open fault tree →P0300 random/multiple misfire
System-level misfire flow: air leaks, ignition baseline, fuel pressure, injector balance, timing plausibility and EGR.
Open fault tree →P0420 catalyst efficiency
Rule out misfires, exhaust leaks and fueling issues, then confirm O2 sensor patterns before condemning the catalytic converter.
Open fault tree →Overboost (P0234 / boost too high)
VNT/wastegate control flow: actuator movement, vacuum/pressure routing, solenoids, boost sensor plausibility and limp behaviour.
Open fault tree →Coolant loss / pressure test workflow
Find external leaks, heater core issues, cap faults or combustion gas intrusion using a structured pressure-test workflow.
Open fault tree →Misfire (cylinder-specific)
Step-by-step isolation for P0301–P0308 or when one cylinder dominates: ignition, injector, compression/leakdown and wiring checks.
Open fault tree →P0401 / P0402 EGR flow (insufficient/excess)
Confirm EGR command vs actual, rule out DP sensor issues, find leaks/blocked passages, and verify MAF/MAP plausibility.
Open fault tree →P0101 MAF performance
Air leaks, contaminated MAF, aftermarket intakes, MAP/MAF plausibility, and trims-based confirmation before replacing parts.
Open fault tree →P0128 coolant thermostat (below temp)
Sensor plausibility, thermostat stuck-open logic, fan control issues, and when a cooling system fault can trigger P0128.
Open fault tree →U0100 communication (lost ECM/PCM)
Network-first triage: power/grounds, CAN physical layer checks, fuse/relay feeds, and module-wake patterns.
Open fault tree →P0011 / P0014 VVT performance
Oil quality/pressure, VVT solenoid & screens, cam/crank plausibility, wiring, and timing checks.
Open fault tree →P013x O2 / lambda sensor circuit & heater
Separate O2 sensor/heater wiring faults from true fueling issues using live data + simple circuit tests.
Open fault tree →P2101 / P2135 throttle & APP correlation
ETC limp mode flow: throttle body condition, connector/wiring, pedal correlation, and relearn/adaptation.
Open fault tree →P0087 low fuel rail pressure
Supply side vs high-pressure side logic: filter, pump, regulator, sensor plausibility, and load-specific failure patterns.
Open fault tree →U0121 lost communication with ABS
ABS module-first triage: fuses, ignition feed, grounds, CAN physical checks, and water ingress patterns.
Open fault tree →P2002 DPF efficiency below threshold
DPF pressure sensor and hose validation, regen prerequisites, and common soot-loading root causes.
Open fault tree →P2563 turbo actuator position / boost control
Compare commanded vs actual, vacuum/e-actuator movement, wiring and adaptation checks.
Open fault tree →P2187 system too lean at idle
Idle-leak vs fueling logic: smoke test, PCV, purge valve, MAF plausibility and trim patterns.
Open fault tree →P2196 O2 sensor signal stuck rich
Separate true rich running from sensor bias, fuel pressure issues, purge faults, and injector leakage.
Open fault tree →U0140 lost communication with BCM
BCM power/ground checks, network physical layer, water ingress patterns, and module wake-up issues.
Open fault tree →More common fault trees
- P2002 DPF efficiency below threshold
- P2563 turbo actuator position
- P2187 system too lean at idle
- P2196 O2 sensor stuck rich
- U0140 lost communication with BCM
- P2106 forced limited power (ETC/throttle)
- P0117 / P0118 coolant temp sensor circuit
- P0700 transmission MIL request (read TCM codes)
- U0101 lost communication with TCM
- P2458 DPF regeneration duration
- P0016 crank/cam correlation
- P2270 O2 signal stuck lean
- P0500 vehicle speed sensor
- P0606 ECU/PCM processor fault
- U0073 CAN bus off
Related workshop guides
What to do next
Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.
Find another symptom flow
Jump to the symptom selector to locate the closest decision tree.
Workshop Guides
Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more.
Diagnostic Codes
Look up DTC meanings, common causes, and related checks.
AI Tools
Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.
P0234 Overboost
Boost too high. Check actuator movement, vacuum/pressure lines, and sensor plausibility.
P0299 Underboost
Boost too low. Smoke test charge air, check diverter/recirc, and boost control.
P2015 Intake Runner Position
Runner flap/actuator plausibility. Separate mechanical stops from wiring/sensor issues.
P2293 Fuel Pressure Regulator Performance
Commanded vs actual fuel pressure mismatch. Verify control, wiring, and supply.
U0415 Invalid Data from ABS/ESP
ABS/ESP plausibility fault. Check wheel speed signals, power/grounds, coding, and CAN.