Code 191

Indicates left or front HO2S detected a problem and adaptive fuel lean limit has been reached.

Possible causes for these codes are:

• Incorrect fuel pressure.
• Faulty fuel injector or circuit.
• Leaking or restricted air intake.
• Faulty MAP sensor.
• Restricted air or fuel filter.

If other codes are present, repair as necessary. If no other codes are present, go to next step.

2) Check Fuel Pressure  Release fuel system pressure. With ignition off, install fuel pressure gauge. Ensure manifold vacuum is connected to fuel pressure regulator. Start engine and allow to idle. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel system pressure is not within specification, repair as necessary. See FUEL SYSTEM in I - SYS/COMP TESTS article in this section. If fuel system pressure is within specification, go to next step.

3) Check System Ability To Hold Fuel Pressure  With fuel pressure gauge installed, cycle ignition from OFF to ON position 3-4 times to pressurize fuel system (DO NOT start engine). If fuel pressure remains at specification for 60 seconds, go to step 4) (MAP/BARO equipped vehicles) or step 6) (all other vehicles). If fuel pressure does not remain at specification for 60 seconds, repair fuel system as necessary. See FUEL SYSTEM in I - SYS/COMP TESTS article in this section.

4) Check MAP/BARO Frequency  Turn ignition off. Connect MAP/BARO tester to MAP/BARO sensor. See Figure in CIRCUIT TEST DF. Turn ignition on. If MAP/BARO voltage is within specification, go to step 5) (MAP/BARO equipped vehicles) or step 6) (all other vehicles). See MAP SENSOR VOLTAGE OUTPUT table. If voltage is not within specification, check wiring to MAP/BARO sensor for corrosion and other damage. Repair as necessary. Clear KAM, and repeat QUICK TEST. If wiring is okay, replace MAP/BARO sensor. Clear KAM, and repeat QUICK TEST.

5) Cylinder Balance Test (MFI)  Connect a tachometer to engine. Start engine, and run it at idle. Disconnect and reconnect injectors individually, noting RPM drop as each injector is disconnected. IAC motor will attempt to re-establish RPM. If each injector produces a momentary drop in RPM, original fault is in ignition system. If each injector does not produce a momentary drop in RPM, go to step 7).

6) Cylinder Balance Test (SFI)  Perform KOER SELF-TEST. After last code is displayed, wait 5-10 seconds. Enter CYLINDER BALANCE TEST by lightly goosing throttle (not WOT). Test time is about 2-3 minutes. To interpret codes retrieved from test, refer to CYLINDER BALANCE TEST SERVICE CODES table. If Code 90 is present, original fault is in ignition system. If Code 90 is not present, go to step 8).

7) Check Injector & Circuit Resistance (MFI)  Turn ignition off. Wait 10 seconds. Disconnect PCM 60-pin connector. Inspect pins for damage. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure resistance of injector bank No. 1 between test pins No. 37 and 58 at breakout box. Measure resistance of injector bank No. 2 between test pins No. 37 and 59 at breakout box. Record resistances. Refer to INJECTOR BANK RESISTANCE table in this circuit test. If each resistance is within specification, go to step 12). If resistance is not as specified, go to step 9).

8) Check Injector & Circuit Resistance (SFI)  Turn ignition off and wait 10 seconds. Disconnect PCM 60-pin connector. Inspect pins for damage. Install EEC-IV Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Measure and record resistance between suspected injector circuit test pin and test pin No. 37. Refer to INJECTOR BANK RESISTANCE table in this circuit test. If each resistance is within specification, go to step 12). If resistance is not as specified, go to step 9).

9) Check Continuity Of Fuel Injector Circuit  Turn ignition off. Disconnect suspect injector wiring harness connector at injector. Measure resistance between test pins No. 37 and 57 at breakout box and each injector VPWR terminal at wiring harness connector. See Figure-58. Measure resistance between injector test pin(s) at breakout box and same injector circuit terminal at each injector wiring harness connector. If each resistance is less than 5 ohms, go to step 10). If each resistance is 5 ohms or more, repair open circuit. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

10) Check Injector Circuit For Short To Power Or Ground  Turn ignition off. With breakout box installed and PCM disconnected, disconnect suspect fuel injector wiring harness connector. Measure resistance between injector test pin(s) and test pins No. 37, 40, 46, 57 and 60. Measure resistance between injector test pin(s) at breakout box and chassis ground. If each resistance is 10,000 ohms or more, replace injector per cylinder balance test service code and repeat QUICK TEST (SFI) or go to next step (MFI). If each resistance is less than 10,000 ohms, repair short. Remove breakout box, reconnect all components and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

11) Isolate Faulty Injector Circuit  Turn ignition off. With PCM disconnected, disconnect all injectors on suspect bank. Connect one injector and measure resistance between test pins No. 37 and 58 (injector bank No. 1) or 59 (injector bank No. 2). Disconnect injector, and repeat process for all other injectors. See INDIVIDUAL INJECTOR RESISTANCE table. If all injectors are within specification, go to step 12). If injectors are not as specified, replace injector. Remove breakout box, and reconnect components. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

12) Check Injector Drive Signal  With ignition off, connect PCM to breakout box. Use a non-powered 12-volt test light, and connect as follows: