P-Codes — Powertrain
Diagnostic Trouble Codes

These codes cover the sensors and systems that control the air/fuel mixture — MAF sensors, MAP sensors, O2 sensors, and fuel trim calculations. They are some of the most misdiagnosed codes because they often point to a root cause upstream of the sensor that triggered them.

The most important diagnostic tool here is fuel trim data. Short-term fuel trim (STFT) shows what the ECU is correcting right now. Long-term fuel trim (LTFT) shows the accumulated learned correction. Consistent positive trims (+10% or more at idle) indicate a lean condition — usually a vacuum leak, failing MAF, or low fuel pressure. Consistent negative trims indicate rich running — injector leak, high fuel pressure, or upstream O2 issue.

Misfire codes are triggered when the PCM detects that one or more cylinders failed to fire correctly. P0300 means multiple or random cylinders — P0301–P0308 identify a specific cylinder. The cause can be ignition-related (plugs/coils), fuel-related (injectors/pressure), air-related (vacuum leaks or compression), or mechanical (bent valve, worn rings).

The coil swap test is the fastest diagnosis: swap the coil from the misfiring cylinder to a known-good cylinder. If the misfire moves with the coil, the coil is bad. If the misfire stays on the same cylinder, the problem is the spark plug, injector, or a compression issue on that cylinder.

This is the range most drivers encounter — catalyst efficiency codes (P0420/P0430), EGR flow codes, and EVAP leak codes. These are the most commonly misdiagnosed codes in this whole range because they’re often blamed on the expensive part (catalytic converter, EGR valve) when the real cause is cheaper to fix.

For EVAP codes: always start with the gas cap. A loose or cracked cap causes P0442 and P0455 in a significant percentage of cases and costs nothing to fix. For P0420: exhaust leaks near the upstream O2 sensor are the second most common cause after a genuinely degraded converter — and much cheaper to repair.

Timing codes in this range cover variable valve timing (VVT) system faults and cam/crank correlation errors. These codes are almost always oil pressure dependent — the most common root cause is low oil level, degraded oil viscosity, or a clogged VVT solenoid screen. Check oil before spending money on parts.

P0016 (cam/crank correlation) in particular can indicate a stretched timing chain on high-mileage engines with poor oil change history. If P0016 appears with a rattling noise on startup that clears when warm, have the timing chain inspected before driving further.

Injector circuit codes (P0201–P0208) indicate the ECU has detected a fault in a specific injector’s electrical circuit — open circuit, short to ground, or resistance out of specification. A fuel smell, rough idle on one cylinder, or hydrolocked engine are signs a leaking injector is involved.

Before replacing an injector, inspect the wiring connector at the injector itself — corrosion, broken pins, and chafed wires are common and far cheaper to fix than a new injector. Test injector resistance with a multimeter — a healthy injector typically reads 12–16 ohms on most port injection systems (varies by manufacturer).

P0700 is a generic alert — it means the Transmission Control Module (TCM) has logged its own fault codes and is requesting MIL illumination. P0700 alone tells you very little. You must read the companion transmission codes (often P0730–P0780 range for shift and solenoid faults, or P0790s for pressure control) to diagnose the actual problem.

Many transmission codes respond well to a fluid and filter service if the fluid is dark or burnt-smelling. Shift solenoids are the most common individual component failure. If the vehicle is in limp mode (fixed gear, limited speed), don’t drive it further until the specific fault is diagnosed.