Diagnostics

Mustang Check Engine Light Codes

Start with common 1996-and-newer Mustang OBD-II powertrain codes, grouped by system. A code points to the system or test that failed; it does not automatically identify the part to replace.

Before You Clear a Code

Code Groups

P0100-P0119

Air Metering & Sensor Input Codes

Mass-airflow, intake-air-temperature, and manifold sensor faults that can make the PCM calculate the wrong fuel and spark strategy.

 P0420-P0434

Catalyst Efficiency Codes

Catalyst monitor codes that compare upstream and downstream oxygen-sensor activity to judge converter efficiency.

 P0115-P0119, P0125-P0128

Cooling & Temperature Codes

Coolant temperature, warm-up, and thermostat codes that can affect emissions, driveability, and overheat diagnosis.

 P0400-P0419

EGR & Secondary Air Codes

Exhaust-gas-recirculation and secondary-air monitor faults found mostly on older OBD-II Mustangs.

 P0440-P0463

EVAP Leak & Purge Codes

Evaporative-emissions leak, purge, vent, and pressure-sensor faults that often start with a loose cap or damaged vapor hose.

 P0190-P0193, P0230-P0232

Fuel Pressure & Delivery Codes

Fuel-pressure sensor, pump, and rail-pressure faults that can overlap with lean, rich, and misfire symptoms.

 P0170-P0175, P2195-P2198

Fuel Trim & Mixture Codes

Lean or rich mixture codes usually mean the PCM has reached the limit of how much fuel it can add or subtract to keep the engine in range.

 P0300-P0316

Misfire Codes

Random and cylinder-specific misfire codes caused by ignition, fuel delivery, compression, or air-leak problems.

 P0130-P0167

Oxygen Sensor & Heater Codes

Front and rear oxygen-sensor circuit, response, and heater faults that affect fuel control or catalyst monitoring.

 P0505-P0507, P2100-P2112

Throttle & Idle Control Codes

Idle-air-control faults on cable-throttle cars and electronic-throttle faults on later Mustangs.

Common Code Pages

P0101 Mass Air Flow Sensor Range/Performance The PCM sees mass-airflow data that does not match expected engine load. P0113 Intake Air Temperature Sensor Circuit High The PCM sees an intake-air-temperature signal that is higher than the normal circuit range. P0128 Coolant Thermostat Below Regulating Temperature The engine is taking too long to reach the expected coolant temperature. P0133 Bank 1 Sensor 1 Oxygen Sensor Slow Response The upstream oxygen sensor on bank 1 is switching more slowly than expected. P0135 Bank 1 Sensor 1 Oxygen Sensor Heater Circuit The upstream bank 1 oxygen-sensor heater circuit is not operating as expected. P0153 Bank 2 Sensor 1 Oxygen Sensor Slow Response The upstream oxygen sensor on bank 2 is switching more slowly than expected. P0155 Bank 2 Sensor 1 Oxygen Sensor Heater Circuit The upstream bank 2 oxygen-sensor heater circuit is not operating as expected. P0171 System Too Lean, Bank 1 The PCM is adding too much fuel on bank 1 because the mixture is lean. P0172 System Too Rich, Bank 1 The PCM is subtracting too much fuel on bank 1 because the mixture is rich. P0174 System Too Lean, Bank 2 The PCM is adding too much fuel on bank 2 because the mixture is lean. P0191 Fuel Rail Pressure Sensor Range/Performance The PCM sees fuel-rail pressure data outside the expected range for operating conditions. P0231 Fuel Pump Secondary Circuit Low The PCM sees low voltage in the fuel-pump secondary circuit. P0300 Random/Multiple Cylinder Misfire Detected The PCM detects misfires that are not isolated to one cylinder. P0301 Cylinder 1 Misfire Detected The PCM detects a misfire on cylinder 1. P0302 Cylinder 2 Misfire Detected The PCM detects a misfire on cylinder 2. P0303 Cylinder 3 Misfire Detected The PCM detects a misfire on cylinder 3. P0340 Camshaft Position Sensor Circuit The PCM sees an electrical or signal fault in a camshaft position sensor circuit. P0401 EGR Flow Insufficient The PCM commands EGR but does not see enough exhaust-gas flow. P0402 EGR Flow Excessive The PCM sees more EGR flow than expected. P0420 Catalyst System Efficiency Below Threshold, Bank 1 The PCM sees downstream oxygen-sensor behavior that suggests bank 1 catalyst efficiency is below threshold. P0430 Catalyst System Efficiency Below Threshold, Bank 2 The PCM sees downstream oxygen-sensor behavior that suggests bank 2 catalyst efficiency is below threshold. P0442 EVAP System Small Leak Detected The EVAP monitor detects a small leak in the fuel-vapor system. P0455 EVAP System Gross Leak Detected The EVAP monitor detects a large vapor leak. P0456 EVAP System Very Small Leak Detected The EVAP monitor detects a very small vapor leak. P0457 EVAP System Leak Detected/Fuel Cap Loose The PCM detects an EVAP leak pattern often associated with the fuel cap or filler seal. P0505 Idle Air Control System The PCM cannot control idle speed with the idle-air-control system. P0506 Idle Air Control System RPM Lower Than Expected Idle speed is lower than the PCM expects. P0507 Idle Air Control System RPM Higher Than Expected Idle speed is higher than the PCM expects. P2104 Throttle Actuator Control System Forced Idle The PCM has limited throttle control and forced idle as a protective response. P2195 Bank 1 Sensor 1 Oxygen Sensor Signal Biased/Stuck Lean The upstream oxygen sensor signal on bank 1 appears stuck lean or biased lean.

Pre-1996 Mustangs

Earlier cars do not all use the same standardized OBD-II code system. Classic cars are best handled symptom-first, while late Fox and early SN95 cars need Ford EEC self-test guidance.

Carburetor & Choke Driveability

Carbureted Mustangs can stumble, stall, or run rich from choke adjustment, vacuum leaks, clogged jets, or ignition timing issues.

Charging, Grounds & Intermittent Electrical Faults

Many pre-OBD symptoms trace back to weak charging output, poor grounds, corroded connectors, or voltage drops.

Classic No-Start Basics

For 1965-1985 Mustangs, start with the battery, starter circuit, ignition spark, fuel delivery, and timing before replacing major parts.

EEC-IV Self-Test Guidance

Fuel-injected Fox and SN95 Mustangs before OBD-II use Ford EEC-IV self-tests rather than standardized OBD-II P-codes.