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Home >> Mazda >> 2001 >> MX-5 Miata Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0171: Fuel Trim Too Lean >> Diagnosis & Repair Procedure

Diagnosis & Repair Procedure

  1. Ensure FREEZE FRAME DATA has been recorded. Check for related service bulletins. Repair as necessary and go to next step.
  2. Turn ignition off and on. Using scan tool, check for DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If no other DTCs are present and no driveability problems are present, go to next step. If a driveability problem is present, go to step  7. If a misfire DTC is present, go to step  7. If any other DTCs are present, go to appropriate test.
  3. Check if DTC P0171 is present in FREEZE FRAME DATA. If DTC is present, go to next step. If DTC is not present, problem is intermittent. See TROUBLE SHOOTING - NO CODES article.
  4. Start engine and allow to idle. Using scan tool, access PID data for ECT V, MAF V, TP V and VS. Ensure all PID data is within specification. See SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE & 626 article. If all PID data is within specification, go to next step. If any data is not within specification, diagnose and repair as necessary and go to step  17.
  5. Referring to FREEZE FRAME DATA, operate vehicle under same conditions as when DTC was set. Record PID data for ECT V, MAF V, TP V and VS. If all PID data is within specification, go to next step. If any PID data is not within specification, go to applicable test in SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE & 626 article. Diagnose and repair as necessary and go to step  17.
  6. Using scan tool, access FHO2S PID data. Depress accelerator pedal to Wide Open Throttle (WOT) and release to race engine. Scan tool should display FHO2S PID is more than .45 volt during acceleration (rich condition) and less than .45 volt during deceleration fuel cut (lean condition). If voltage is as specified, check for vacuum leaks between air cleaner and throttle body. Check for vacuum leaks at throttle body, intake plenum and intake manifold. Also check for cracked, damaged or loose vacuum hoses. Repair as necessary and go to step  17. If voltage is not as specified, check for exhaust leaks up stream of front HO2S. Repair as necessary. If no exhaust leak is found, replace front HO2S and go to step  17.
  7. Using scan tool, access MAF V PID data. Depress accelerator pedal to Wide Open Throttle (WOT) and release to race engine. Scan tool should display quick voltage changes. If voltage is as specified, go to next step. If voltage is not as specified, replace MAF sensor and go to step  17.
  8. Inspect air intake system and all vacuum lines for vacuum leaks. If no problem is found, go to next step. If problem is found, repair as necessary and go to step  17.
  9. Check fuel line pressure. See BASIC DIAGNOSTIC PROCEDURES article. If fuel line pressure is 53-61psi (3.7-4.3 kg/cm2 ), go to step  17. If fuel line pressure is less than specified, go to next step. If fuel line pressure is more than specified, check fuel pump maximum pressure. See BASIC DIAGNOSTIC PROCEDURES article. Check fuel return system for plugging. Repair as necessary. If no problem is found, replace fuel pressure regulator and go to step  17.
  10. Check fuel pump maximum pressure. See BASIC DIAGNOSTIC PROCEDURES article. If maximum pressure is about 92 psi (6.4 kg/cm2 ), go to next step. If maximum pressure is not about 92 psi (6.4 kg/cm2 ), repair fuel pump wiring. If fuel pump wiring is okay replace fuel pump and go to step  17.
  11. Check for leaks in fuel system down stream of fuel pump. If any leaks are found, repair as necessary. If no leaks are found, check for plugging at fuel filter and fuel inlet filter (sock). If plugging is present, check fuel tank for debris. If fuel tank is okay, replace fuel filter and go to step  17. If fuel tank has debris, clean tank, replace fuel inlet filter (sock) and fuel filter. If fuel tank, fuel filter and fuel inlet filter (sock) are okay, replace fuel pressure regulator and go to step  17.
  12. Check primary and secondary ignition system. If both primary and secondary ignition system is okay, go to step  15. If secondary ignition system is not operating properly, repair or replace as necessary. If primary ignition system is not operating properly, go to next step.
  13. Disconnect ignition coil connector. Turn ignition on. Measure voltage at coil harness connector terminal "A" (Black/White wire). If battery voltage is present, go to next step. If battery voltage is not present, repair open in Black/White wire between coil and ignition switch and go to step  17.
  14. Check ignition coil resistance. See BASIC DIAGNOSTIC PROCEDURES article. If coil is okay, go to next step. If coil is faulty, replace coil and go to step  17.
  15. Check engine compression. Engine compression should be a minimum of 146 psi (10.22 kg/cm2 ) at 300 RPM with a maximum variation of 28 psi (2 kg/cm2 ). If compression is as specified, go to next step. If compression is not as specified, repair as necessary and go to step  17.
  16. Check fuel injector operation at suspect cylinder(s). See applicable test in SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE & 626 article. If no problem is found, go to next step. If problem is found, replace injector(s) as necessary and go to next step.
  17. Reconnect all connectors. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Perform PCM adaptive memory drive mode, EGR monitor repair verification drive mode and HO2S/TWC monitor repair verification drive mode. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM. Check for DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If DTC P0171 is present, replace PCM and go to next step. On vehicles equipped with immobilizer system, reprogram ignition key identification numbers. See COMPUTER RELEARN PROCEDURES article in GENERAL INFORMATION. If DTC P0171 is not present, go to next step.
  18. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If no other DTCs are present, testing is complete. If any other DTCs are present, go to applicable test and repair.
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