Fault Code Guides
DTC guides — evidence, not parts cannon
Each fault-code guide explains what the code means, the system involved, why the code alone does not identify the failed component, and the next-best-test decision tree to confirm the root cause before any part is replaced.
- P0087Fuel Delivery / Direct InjectionP0087: Fuel Rail Pressure Too Low — Diagnosis, Causes & TestingP0087 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.
- P0300Ignition / Fuel / MechanicalP0300: Random/Multiple Cylinder Misfire Detected — Diagnosis, Causes & TestingP0300 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.
- P0299Forced Induction / Air ChargeP0299: Turbo/Supercharger Underboost — Diagnosis, Causes & TestingP0299 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.
- P0171Fuel / Air / InductionP0171: System Too Lean (Bank 1) — Diagnosis, Causes & TestingP0171 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.
- P0172Fuel / Air / InductionP0172: System Too Rich (Bank 1) — Diagnosis, Causes & TestingP0172 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.
- P0191Fuel Delivery / Direct InjectionP0191: Fuel Rail Pressure Sensor Range/Performance — Diagnosis, Causes & TestingP0191 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.
- P0335Engine Speed / Position SensingP0335: Crankshaft Position Sensor Circuit — Diagnosis, Causes & TestingP0335 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.
- P0340Engine Position / Valve TimingP0340: Camshaft Position Sensor Circuit — Diagnosis, Causes & TestingP0340 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.
- P0562Charging / Electrical PowerP0562: System Voltage Low — Diagnosis, Causes & TestingP0562 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.
- P0700Transmission Control / Powertrain CommunicationP0700: Transmission Control System Malfunction — Diagnosis & Next StepsP0700 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.