The searchable automotive diagnostic knowledge hub

P0087 means actual fuel rail pressure is below commanded pressure. Learn the evidence-driven diagnostic workflow — desired vs actual, supply, HPFP, sensor, regulator, injector — and why the code alone does not name the failed part.

P0300 means the ECM detected random or multiple-cylinder misfires. Learn to separate random from cylinder-specific misfires using counters, fuel trims, ignition, injectors, compression and vacuum leaks.

P0299 means actual boost is below commanded boost. Learn the evidence-driven workflow — requested vs actual boost, charge-air leaks, wastegate/VGT, vacuum control, MAP sensing, exhaust restriction and turbo condition.

P0171 means fuel trim has reached its lean correction limit. Learn STFT/LTFT interpretation, MAF plausibility, intake leaks, fuel pressure, injectors, purge and O2 feedback — and how trims change with RPM and load.

P0172 means fuel trim has reached its rich-correction limit. Learn STFT/LTFT interpretation, MAF over-reporting, fuel pressure, leaking injectors, purge, O2 feedback and coolant-sensor plausibility.

P0191 means the FRP sensor signal is implausible or out of range. Learn to separate a sensor fault from a genuine pressure problem using requested vs actual, mechanical-gauge correlation, wiring, reference and ground.

P0335 means the ECM cannot reliably read the crankshaft position signal. Learn signal integrity, power/ground, cranking RPM, crank/cam correlation, reluctor/trigger faults and oscilloscope diagnosis.

P0340 means the ECM cannot reliably read the camshaft position signal. Learn signal integrity, power/ground, crank/cam correlation, reluctor/trigger faults, VVT interaction and oscilloscope diagnosis.

P0562 means ECM supply voltage is below threshold. Learn battery state, charging system, voltage drop, grounds, terminal integrity, alternator output and module supply — and why low voltage drives cascading false codes.

P0700 is an indicator that the transmission control module has stored faults. Learn why technicians must interrogate the TCM directly rather than replace parts based on P0700 alone.

Evidence-driven BMW diagnostic workflow: ISTA vehicle test, fault memory, live data, requested vs actual, CAS/DME/DDE, no-start, fuel pressure, battery/charging and communication faults. Not affiliated with BMW.

Evidence-driven Audi/VW diagnostic workflow: VCDS Auto-Scan, fault-code interpretation, freeze-frame, measuring values, requested vs actual, FSI/TFSI, boost, CAN and electrical diagnosis. Not affiliated with VAG.

Evidence-driven Audi B8 (A4/S4/S5/RS5) diagnostic workflow: VCDS Auto-Scan, gateway, fault memory, freeze frame, measuring values, requested vs actual, FSI/TFSI fuel pressure, CAN, electrical, no-start, misfire and charge faults. Not affiliated with VAG.

Evidence-driven Audi 8V platform diagnostic workflow: VCDS Auto-Scan, gateway, fault memory, measuring values, requested vs actual, TFSI fuel pressure, boost, CAN, electrical, low-voltage, sensor plausibility, repair verification. Not affiliated with VAG.

Evidence-driven VAG MQB diagnostic workflow: gateway, control-module communication, VCDS Auto-Scan, measuring values, requested vs actual, CAN, electrical supply, ground faults, boost/fuel, misfire, module-level vs network-wide faults, repair verification. Not affiliated with VAG.

Evidence-driven Audi CAUA 4.2 FSI V8 diagnostics: engine identification, fuel-pressure requested vs actual, air/load plausibility (naturally aspirated), misfire, crank/cam, electrical supply, VCDS workflow, repair verification. Not affiliated with VAG.

Evidence-driven VAG 2.0 TFSI diagnostics: variant warning, boost requested vs actual, fuel pressure requested vs actual, air/fuel, misfire, starting/timing/correlation, electrical, VCDS workflow, repair verification. Not affiliated with VAG.

Evidence-driven Audi S5 CAUA low fuel pressure diagnosis: actual vs requested rail pressure, low-side supply vs high-pressure generation, sensor plausibility, control vs mechanical, load and hot/cold evidence. Not affiliated with VAG.

Evidence-driven Audi S5 CAUA long crank diagnosis: hot vs cold vs after-standing, cranking speed, rail-pressure build, residual pressure, crank/cam sync, injector command, temperature-dependent evidence. Not affiliated with VAG.

Evidence-driven Audi S5 CAUA rough idle diagnosis: misfire counters, fuel trims, air measurement, intake leakage, cylinder-specific mechanical, crank/cam correlation, sensor plausibility. Not affiliated with VAG.

Evidence-driven 2.0 TFSI P0299 underboost diagnosis: requested vs actual boost, charge-air leak, actuator/control, MAP plausibility, turbo mechanical, exhaust restriction. P0299 does not prove turbo failure. Not affiliated with VAG.

Evidence-driven 2.0 TFSI fuel pressure diagnosis: actual vs requested rail pressure, low-side supply vs high-pressure generation, sensing vs control vs electrical, key-on / cranking / idle / load / hot-restart evidence. Not affiliated with VAG.

Evidence-driven 2.0 TFSI misfire diagnosis: per-cylinder counters, ignition swap, injector balance, fuel pressure, air leakage, fuel trims, compression, crank/cam correlation, sensor/load plausibility. Not affiliated with VAG.

Evidence-driven 2.0 TFSI rough idle diagnosis: rough idle with misfire vs without, mixture trims, airflow/MAF, intake leakage, injector, fuel pressure, compression, crank/cam correlation. A rough idle is a symptom, not a component. Not affiliated with VAG.

Evidence-driven Audi 3.0 TFSI (EA837 supercharged V6) misfire diagnosis: isolated vs bank vs multiple vs random, ignition, injector, fuel pressure, mixture, airflow, mechanical, timing-chain/correlation. Not affiliated with Audi / VAG.

Evidence-driven Audi 3.0 TFSI low power diagnosis: load request vs achieved, airflow, pressure/load sensor, fuel pressure, mixture, throttle/load control, ignition/misfire, electrical, mechanical, limp strategy. Reduced power is a state, not a diagnosis. Not affiliated with Audi / VAG.

Evidence-driven Audi/VW 3.0 TDI long crank diagnosis: cranking condition, engine-speed signal, requested vs actual rail pressure, low-side supply, pressure control, crank/cam sync, temperature, electrical, mechanical. Not affiliated with VAG.

Evidence-driven Audi/VW 3.0 TDI low fuel pressure diagnosis: low-side supply vs high-pressure generation vs regulation vs sensor plausibility vs electrical vs leakage. Low rail pressure is a system condition, not proof of HPFP failure. Not affiliated with VAG.

Evidence-driven BMW M57 long crank diagnosis: cranking voltage/speed, requested vs actual rail pressure, low-side supply, crank/cam sync, temperature-dependent evidence. Diesel common-rail. Not affiliated with BMW.

Evidence-driven BMW M57 low fuel pressure diagnosis: requested vs actual rail pressure, low-side supply vs high-pressure generation, control vs sensor vs electrical vs leakage. Diesel common-rail. Not affiliated with BMW.

Evidence-driven BMW N47 crank-no-start diagnosis: cranking voltage, engine-speed signal, crank/cam sync, rail-pressure build, injector authorization, electrical, mechanical. True no-start, not long-crank. Not affiliated with BMW.

Evidence-driven BMW N47 low power diagnosis: boost/load, air-mass, charge-air integrity, turbo control, pressure-sensor plausibility, fueling, limp-mode strategy. Low boost does not prove turbo failure. Not affiliated with BMW.

Evidence-driven BMW N55 misfire diagnosis: single vs multiple vs random misfire, ignition, injector, fuel pressure, air leakage, fuel trims, Valvetronic/VANOS context, compression. Not a parts-swap list. Not affiliated with BMW.

Evidence-driven BMW N55 low boost diagnosis: requested vs actual boost, charge-air integrity, wastegate/VANOS control, MAP plausibility, airflow, exhaust restriction. Low boost does not condemn the turbo. Not affiliated with BMW.

Evidence-driven BMW N20 rough idle diagnosis: cylinder-specific misfire, mixture-related, airflow, mechanical imbalance, unstable idle without misfire, fuel trims, Valvetronic/VANOS context. Not affiliated with BMW.

Evidence-driven BMW N20 low boost diagnosis: requested vs actual boost, charge-air leaks, wastegate/actuator control, MAP plausibility, airflow, exhaust restriction. P0299 does not prove turbo failure. Not affiliated with BMW.

Evidence-driven BMW E90 3 Series diagnostic workflow: ISTA vehicle test, all-module fault memory, CAS/DME/DDE, requested vs actual live data, K-CAN/PT-CAN, FRM/JBE power distribution, no-start and charge faults. Not affiliated with BMW.

Evidence-driven BMW F30 3 Series diagnostic workflow: ISTA, FEM body domain, FlexRay/EN-ENET, B48/N20/N55 DME, requested vs actual live data, CAS-4 start authorisation, charge and network faults. Not affiliated with BMW.

Evidence-driven BMW E70 X5 diagnostic workflow: ISTA vehicle test, all-module fault memory, M57/N57 diesel and N55 petrol, CAS, FRM/JBE, VTG transfer box, DSC, charge and network faults. Not affiliated with BMW.

Evidence-driven BMW F10 5 Series diagnostic workflow: ISTA, FEM body domain, N57/N47 diesel and N20/N55 petrol, CAS-3/CAS-4, requested vs actual live data, DSC/EGS derate and network faults. Not affiliated with BMW.

Evidence-driven BMW M57 diagnostics: DDE fault memory, low-pressure supply, high-pressure common-rail rail pressure requested vs actual, crank/cam synchronisation, injector contribution, air-mass, boost control, EGR, ISTA workflow, live data and oscilloscope testing. Not affiliated with BMW.

Evidence-driven BMW N47 diagnostics: DDE architecture, common-rail rail pressure, crank/cam synchronisation, air-mass, boost control, EGR/air-path, electrical supply, start/no-start, rough running, reduced power, ISTA workflow, live data and oscilloscope. Teaches evidence-based timing differentiation. Not affiliated with BMW.

Evidence-driven BMW N55 diagnostics: DME architecture, direct-injection low/high fuel pressure, rail-pressure plausibility, Valvetronic context, VANOS/cam correlation, boost and charge-air, wastegate control, MAF/MAP, fuel trims, misfire differentiation, electrical supply, ISTA workflow, live data and oscilloscope. Not a common-problems list. Not affiliated with BMW.

Evidence-driven BMW N20 diagnostics: DME architecture, direct-injection low/high fuel pressure, boost control, Valvetronic, VANOS/cam correlation, fuel trims, air-path, misfire differentiation, electrical supply, ISTA workflow, live data and oscilloscope. Teaches evidence-based workflow, not stereotypical failure lists. Not affiliated with BMW.

BMW-specific cranks-but-won't-start workflow: cranking quality, engine-speed signal, DME/DDE communication, CAS/FEM/BDC start authorisation, immobiliser enable, fuel supply, common-rail pressure, ignition, injector command, crank/cam sync, compression. ISTA vehicle test and next-best-test logic. Not affiliated with BMW.

BMW long crank diagnosis differentiated by condition: cold long crank, hot long crank, first-start-after-sitting, short hot-soak restart. Fuel-pressure build-up, pressure retention/bleed-down, cranking speed, engine-speed signal, injector delivery, start authorisation. ISTA evidence comparison. Not affiliated with BMW.

BMW low fuel rail pressure diagnosis: requested vs actual, low-pressure supply vs high-pressure generation, cranking/idle/load context, pressure sensor plausibility, reference/ground/wiring, metering/control valve, injector leak-off, filter restriction, pump control, DTC relationships, decisive test progression, verification. No universal rail-pressure values. Not affiliated with BMW.

BMW reduced engine power diagnosis: determine which control strategy triggered the derate — boost, fuel pressure, airflow, throttle/Valvetronic, VANOS, sensor plausibility, exhaust restriction, low voltage, thermal protection, transmission. Read the derate reason, don't guess the component. Not affiliated with BMW.

BMW underboost diagnosis: requested vs actual boost, MAP plausibility, MAF relationship, charge-air leak testing, vacuum control, wastegate/VGT, turbo mechanical condition, exhaust restriction, control solenoid/actuator, power/ground/wiring, smoke/pressure testing, next-best-test. Not affiliated with BMW.

BMW rough idle diagnosis: misfire counters, fuel trims, injector contribution, ignition, vacuum/intake leaks, MAF/MAP plausibility, VANOS/Valvetronic context, compression/mechanical, mounts as vibration transmitters, temperature-dependent behaviour, live data, scope. Not affiliated with BMW.

BMW misfire diagnosis: single-cylinder vs random/multiple, ignition, injector/fuel, air leak, compression, timing, mechanical, swap-test strategy, cylinder contribution/misfire counters, fuel trims, relative compression, oscilloscope, DME evidence, why a fault code alone does not prove coil/injector failure. Not affiliated with BMW.

BMW undervoltage / low-voltage fault diagnosis: battery state, cranking voltage, charging system, IBS context, voltage drop, grounds, terminal integrity, alternator output, module supply voltage, low-voltage cascades across modules, communication faults from supply instability. No universal numeric voltage thresholds. Not affiliated with BMW.

BMW CAS / DME-DDE start authorisation diagnosis: immobiliser role, CAS/FEM/BDC by generation, DME/DDE start permission, key recognition, module communication, synchronisation/alignment, why crank/no-start occurs with healthy fuel/mechanical, scan-tool evidence, differentiating authorisation from rail-pressure and crank-signal faults. Legitimate repair only. Not affiliated with BMW.

BMW CAN communication fault diagnosis: network topology, multiple bus systems, gateway/module relationships, no-communication vs stored DTC, low voltage as a network fault cause, power/ground checks, open/short, branch isolation, termination/topology, oscilloscope, network-wide vs one-module, common mistakes. No unverified pinouts. Not affiliated with BMW.

How VCDS fits an evidence-driven workflow: Auto-Scan, control modules, DTCs, freeze-frame, measuring values, requested vs actual, logging and exporting evidence into MechanIQ. Not affiliated with Ross-Tech.

How ISTA fits an evidence-driven BMW workflow: vehicle test, control-unit tree, fault memory, test plans, measurement values, requested vs actual and DME/DDE/CAS context. Not affiliated with BMW.

How Delphi / Autocom fits an evidence-driven multi-brand workflow: system scan, DTCs, live data, actuator tests, vehicle-identification limits, exporting results and how MechanIQ uses the evidence. Not affiliated.

How to read fuel rail pressure live data: requested vs actual, low-pressure supply vs high-pressure generation, cranking vs idle vs load, sensor/wiring faults, control-valve influence and deviation analysis.

How to read fuel trims: STFT, LTFT, bank differences, idle vs elevated RPM, load influence, vacuum leaks, MAF bias, fuel pressure, injectors, purge and O2 feedback — and why one trim value is not the root cause.

How to read MAF live data: air-mass plausibility vs RPM/load, relationship with MAP/throttle/fuel trims, contamination, intake leaks, under vs over-reading, and why replacing a MAF from a DTC alone is poor diagnosis.

How to read MAP / boost live data: requested vs actual, barometric and key-on plausibility, MAP sensor bias, boost leaks, turbo/VGT/wastegate, charge-air restrictions and the link to P0299.

How to read battery and charging voltage live data: resting state, cranking voltage, charging behaviour, module supply voltage, voltage drop, ground integrity, low-voltage DTC cascades and the link to P0562.

How to diagnose a 5V reference fault: shared circuits, multiple-sensor DTCs, shorted sensor pulling down the reference, open/short-to-ground/short-to-voltage, disconnect-and-observe isolation, meter and scope testing.

Why voltage-drop testing matters: power-side and ground-side drop, high resistance, corroded terminals, poor crimps, battery cables, starter/charging and module supply — and why unloaded continuity passes while the circuit fails under load.

How to diagnose CAN bus faults: CAN-H and CAN-L, termination, dominant/recessive signalling, shorts, opens, module branch faults, network-wide vs branch-specific loss, resistance and oscilloscope testing.

How to test grounds properly: ground path, loaded ground testing, voltage drop, shared grounds, ground offset, sensor-ground vs chassis-ground, module grounds, intermittent grounds and wiggle/load testing.

How to scope a crank sensor: Hall vs magnetic/VR concepts, missing-tooth patterns, signal dropout, amplitude behaviour, intermittent wiring, air-gap influence and why RPM data alone does not prove waveform integrity.

How to scope crank vs cam correlation: synchronisation concept, timing relationship, mechanical timing faults, sensor dropout, trigger-wheel faults, timing-chain/belt shift — and why both signals can look fine individually but correlate incorrectly.

How to scope a CAN bus: CAN-H/CAN-L complementary signalling, differential concept, ringing/noise, short-to-ground/voltage appearance, missing communication, branch faults, termination and why resistance and waveform testing complement each other.

A symptom-first framework for cranks-but-won't-start: battery/cranking, immobiliser, engine speed, fuel supply, rail pressure, ignition, injector command, compression, timing and air path — with links to P0335, P0340, P0087.

A symptom-first framework for long crank: cold vs hot, fuel-pressure build, pressure bleed-down, crank speed, engine-speed signal, injector/fuel delivery, temperature-dependent faults, immobiliser and timing.

A symptom-first framework for rough idle: misfire counters, fuel trims, air leaks, MAF/MAP, ignition, injectors, compression, engine mounts as symptom-not-cause, timing and sensor plausibility.

A symptom-first framework for misfire: single-cylinder vs random, ignition, fuel, compression, air leak, timing, mechanical, swap tests, relative compression and scope — with the link to P0300.

A symptom-first framework for low power: boost, fuel delivery, airflow, exhaust restriction, sensor plausibility, limp strategy, transmission influence and requested vs actual — with the link to P0299.

A symptom-first framework for limp mode: engine vs transmission limp, stored faults, protection strategy, low voltage, boost/fuel, transmission DTCs and why P0700 alone does not identify a transmission component.