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Home >> Mazda >> 2002 >> MX-5 Miata Base, Automatic >> Repair and Diagnosis >> Engine Performance >> Testing & Diagnosis >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0171: Fuel Trim System Too Lean >> Diagnostic Procedure

Diagnostic Procedure

  1. Turn ignition off. Connect New Generation Star (NGS) tester or equivalent to OBD-II Data Link Connector-2 (DLC-2). See Figure. Turn ignition on. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check for service bulletins and/or other line of repair information related to this DTC. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  18. If repair information is not available, go to next step.
  2. Monitor for DTC or pending coderelated to this DTC. If misfire DTC is present, go to step  8. If other DTC is present, follow appropriate DTC diagnostic procedure. If no other DTC is set, but driveability concern is present, go to step  8. If no other DTC is set and no driveability concern is present, go to next step.
  3. Check for triggered DTC on recorded FREEZE FRAME DATA. If malfunction code recordedis P0171, go to next step. If malfunction code recordedis different that P0171, follow appropriate DTC diagnostic procedure.
  4. Start engine and let it idle until it reaches normal operation temperature. Using NGS tester or equivalent, access ECT, MAF, TP, and VSS PIDs. For system PID specifications. See SYSTEMS INPUT SIGNAL SPECIFICATIONS  table.
    SYSTEMS INPUT SIGNAL SPECIFICATIONS

    Application Sensor Voltage At Idle Speed
    ECT
    68°F (20°C) Approximately 3.0 Volts
    176°F (80°C) Approximately 0.9 Volts
    MAF
    Switch on 0.9-2.0 Volts
    At idle-After warm up 1.7-2.4 Volts
    TP
    Close Throttle 0.1-1.1 Volts
    Wide Open Throttle 3.0-4.6 Volts
    VSS
    Voltage Should Moves Slightly Under 5 Volts While Driving

    If all systems input voltages are as specified, go to next step. If any system input voltage is not as specified, inspect system sensor and system wiring harness, repair or replace as necessary. For circuit identification, see MIATA in WIRING DIAGRAMS article. After repair, go to step  18.

  5. Inspect same PIDs while simulating recorded FREEZE FRAME DATA conditions, If no input signal is drastically change, go to next step. If any input signal is drastically change, inspect system sensor and system wiring harness, repair or replace as necessary and go to step  18.
  6. With engine running at idle and at normal operation temperature. Using NGS tester or equivalent, access O2S11 PID. Verify O2S11 PID while racing engine (in PARK). Ensure that each value is within following conditions:
    • More than 0.45 volts. when suddenly depressing accelerator pedal (rich condition).
    • Less than 0.45 volts. just after release of accelerator pedal (lean condition).

    If O2S11 PIDs readings are as specified, go to next step. If any O2S11 PID reading is not as specified, inspect for exhaust leak between exhaust manifold and front H2OS. If exhaust leak is found, repair as necessary and repeat this test, If exhaust leak is not found, replace front H2OS. After repair, go to step  18.

  7. Leave engine running at idle. While spraying rust penetrating agent in short spurts, inspect for air suction (vacuum leak) due to cracks, damages, or loose parts between following components:
    CAUTION: DO NOT spray rust penetrating agent on or near coils or plug wires, an arcing wire or coil could start a fire. Allow enough time for rust penetrating agent to dissipate.
    • From Air cleaner box assembly to throttle body.
    • From throttle body to dynamic chamber.
    • From dynamic chamber to intake manifold.
    • Vacuum hoses.

    If air suction (vacuum leak) is not found, go to next step. If air suction (vacuum leak) is found, repair or replace necessary component and go to step  18.

  8. With engine running at idle and at normal operation temperature. Using NGS tester or equivalent, access MAF PID. While racing engine RPM, verify MAF PID changes quickly according to engine RPM. If MAF PID changes as specified, go to next step. If MAF PID does not changes as specified, replace MAF sensor. After repair, go to step  18.
  9. Turn ignition off and perform fuel pressure test. Release fuel pressure from fuel system. Install fuel pressure gauge. For proper fuel pressure test. See FUEL PRESSURE under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pressure should be 53-61 psi (3.7-4.3 kg/cm2 ). If fuel pressure is as specified, go to step  12. If fuel pressure is not as specified, go to next step.
  10. Turn ignition off. Visually inspect fuel lines for leaks. If fuel leak is found, repair or replace component as necessary and go to step  18. If fuel leak is not found, inspect fuel filter for following condition:
    • Restriction or clogging at fuel filter (high pressure). If restriction or clogging is found. Replace fuel filter and go to step  18.
    • Foreign material or stain in fuel filter (low pressure). If foreign material or stain is found inside fuel filter. Clean fuel tank and replace fuel filter. After repair, go to step  18.
    • If all items above are okay, go to next step.
  11. Turn ignition on. Perform fuel pump maximum pressure test. See FUEL PUMP MAXIMUM PRESSURE INSPECTION under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pump maximum pressure should be 92 psi (6.5 kg/cm2 ). If fuel pressure is not as specified, check as follow:
    • If fuel pump maximum pressure is less than 92 psi (6.5 kg/cm2 ). Check fuel pump circuit for open or poor connection, repair or replace connector or circuit as necessary and go to step  18.
    • If connector and circuits are okay, but fuel pump maximum pressure is less than 92 psi (6.5 kg/cm2 )replace fuel pump and go to step  18.
    • If connector and circuits are okay, but fuel pump maximum pressure is more than 92 psi (6.5 kg/cm2 ), replace fuel pressure regulator and go to step  18.
    • If fuel pump maximum fuel pressure is as specified, go to next step.
  12. Return vehicle to original condition. Start engine and let it idle. Using a timing light, verify blinking condition on each cylinder. If all cylinder show good blinking conditions, go to step  16. If any cylinder show faulty blinking conditions, go to next step.
  13. Turn ignition off. Inspect high-tension leads for installation, condition, corrosion on terminal, open lead or damaged cover. If high-tension leads are okay, go to next step. If any high-tension leads is faulty, replace high-tension lead and go to step  18.
  14. Disconnect ignition coils 3-pin harness connectors. See Figure. Turn ignition on. Measure voltage between ground and Ignition coils harness connectors terminal "A" (Black/White wire). Voltage should be battery voltage. If voltage is as specified, go to next step. If voltage is not as specified, repair open or short (in Black/White wire) between suspected ignition coil and ignition switch. For circuit identification, see MIATA in WIRING DIAGRAMS article. After repair, go to step  18.
  15. Leave ignition coils harness connectors disconnected and disconnect high tension led wires. Check ignition coils primary and secondary resistances. See IGNITION COIL RESISTANCE - MIATA under IGNITION CHECKS in BASIC DIAGNOSTIC PROCEDURES article. If resistances are as specified, go to next step. If resistances are not as specified, replace faulty ignition coil. After repair, go to step  18.
  16. Return vehicle to original condition. Perform engine compression test. For proper engine compression test. See COMPRESSION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES article. Compression should be about 198 psi (13.9 kg/cm2 ) at 300 RPM. If engine compression is as specified, go to next step. If engine compression is not as specified, determine source of low compression. Repair as necessary and go to step  18.
  17. Turn ignition off. Inspect fuel injectors operation. See Fuel Injector test in FUEL CONTROL under FUEL SYSTEM in SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. If fuel injectors are operating properly, go to next step. If any fuel injector is faulty, replace fuel injector and go to next step.
  18. Return vehicle to original condition. Using NGS tester or equivalent, clear DTCs from PCM memory. Start engine and run DRIVE MODE 1, 2 and 3. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM.Monitor for pending DTCs. If same DTC P0171 is present, replace PCM. For PCM removal and installation, see POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION article. If any other DTC is present, follow appropriate DTC diagnostic procedure.
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