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Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. (Fuel-contaminated engine oil may affect Codes 41/172, 91/136, 42/173 and 92/137. If fuel-contaminated engine oil is suspected, remove PCV valve from valve cover and repeat QUICK TEST). If problem is corrected, change engine oil and filter. Only use this test to diagnose:

To prevent replacement of good components, be aware the following non-EEC areas may be cause of driveability concerns:

INDIVIDUAL INJECTOR RESISTANCE

Engine Ohms
Probe 2.0L 11-18
Fig 1: Test Schematic
G93G40572
CIRCUIT TEST H ACRONYMS

Acronym Definition
HO2S Heated Oxygen Sensor
MFI Multiport Fuel Injection
SFI Sequential Multiport Fuel Injection

1) Check For Contaminated Engine Oil  Turn ignition off. Remove PCV valve from valve cover. Inspect PCV system for damage. Inspect valve for blockage and movement of valve plunger. Repair as necessary. Perform KOEO and KOER SELF-TEST. Service ignition continuous memory code (if present) before KOER codes. If vehicle is a no-start, KOER Code 41/172, 42/173, 91/136 or 92/137 or Continuous Memory Code 171, 172, 173, 174, 175, 176, 177, 178, 41/144 or 91/139 is set, install PCV valve and go to step 2). If no codes are set, change engine oil and filter. Install PCV valve, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

2) Check Fuel Pressure  Turn ignition off. Release fuel pressure. Install fuel pressure gauge. Ensure manifold vacuum supply is connected to fuel pressure regulator (if equipped). Run engine at idle and check fuel pressure. If vehicle will not start, cycle ignition on and off several times. Check fuel pressure. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is within specification, go to step 3). If pressure is not within specification, go to FUEL SYSTEM in SYSTEM/COMPONENT TESTS - EEC-IV article in this section.

3) Check System Ability To Hold Fuel Pressure  Turn ignition on. Check fuel system for leaks. Repair or replace as necessary. If fuel system does not leak and pressure remains at specification for 60 seconds, go to step 4) (no-starts), or step 6) (SFI).

4) Fuel Delivery Test  Release fuel system pressure. With fuel pressure gauge installed, pressurize fuel system as in step 1). Disconnect inertia switch. Crank engine 5 seconds. If fuel pressure drop is greater than 5 psi after 5 seconds of cranking, EEC-IV system is not causing no-start condition. Check additional no-start tests in TESTS W/O CODES - EEC-IV article in this section. If fuel pressure drop is less than 5 psi, remove fuel pressure gauge. Connect inertia switch. Go to step 8) for SFI.

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 does not produce a momentary drop in RPM, go to step 7). If each injector produces a momentary drop in RPM, go to step 13) for Codes 41/172, 91/136 or 176, step 24) for Codes 42/173, 92/137 or 177 or step 14) for all others.

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 fault codes are present, go to step 8). Code 77/538 indicates throttle was touched when test was ran and test is not completed. Code 538 could also be output if Continuous Memory Code 214 (CID) is present. If Continuous Memory Code 214 is present, see appropriate chart under SERVICE CODE-TO-TEST MENUS for instruction. For symptoms or Code 41/172, 91/136 or 176, go to step 13). For Code 42/173, 92/137 or 177, go to step 24). For all other codes, go to step 14).

CYLINDER BALANCE TEST SERVICE CODES

Service Code Application
90 Pass
10 Cylinder No. 1
20 Cylinder No. 2
30 Cylinder No. 3
40 Cylinder No. 4
77/538 Retest

7) Check Injector & Harness 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, repair open circuit on VPWR circuit for no-start or go to step 9) for all other conditions.

8) Check Injector & Harness 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. For no-start, measure and record resistance between 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, repair open circuit on VPWR circuit for no-start or go to step 9) for all other conditions.

9) Check Continuity Of Fuel Injector Harness  Turn ignition off. Disconnect injector wiring harness connector at injectors. Set DVOM to 200-ohm scale. Measure resistance between test pins No. 37 and 57 atbreakout box and each injector VPWR terminal at wiring harness connector. See Figure . 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 Harness Ckt For Short To Power Or Ground  Turn ignition off, and wait for 10 seconds. With breakout box installed and PCM disconnected, disconnect fuel injector wiring harness connector. See Figure . 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. Install breakout box, and disconnect PCM. Disconnect all injectors on suspect bank. With DVOM set on 200-ohm scale, 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 and breakout box installed, connect PCM to breakout box. Use a non-powered 12-volt test light, and connect as follows:

Connect test light between test pin No. 37 and suspect injector test pin at breakout box.

Crank or start engine. If test light glows dimly, system is operating correctly. Clean injectors, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST and CYLINDER BALANCE TEST. If test light does not glow dimly (no light/bright light), replace PCM. Reconnect all components, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

13) Check Thermactor Operation  If vehicle is equipped with pulse-air or does not have secondary air injection system, go to step 14). A HO2S which is always lean could be caused by thermactor air being diverted upstream from HO2S. With dual HO2S, Code 41/172 applies to right or rear HO2S and Code 91/136 or 176 applies to left or front HO2S. Turn ignition off. Disconnect secondary air hose(s) from pump and vent to atmosphere. Perform KOER SELF-TEST. If Code 41/172, 91/136 or 176 is present, reconnect thermactor air hoses and go to next step. If code(s) is not present, repair air injection system as necessary.

14) Check HO2S Integrity  A HO2S which is always lean, slow to switch or does not switch could be caused by:

Turn ignition off. Inspect HO2S wiring harness for damage. Inspect HO2S and connector for signs of fluid entry. Repair or replace as necessary. Start engine, and run it at 2000 RPM for 2 minutes. Turn ignition off. Run KOER SELF-TEST. If fault codes are present, go to step 15) (MAP sensor-equipped models) or step 16) (MAF sensor-equipped models). If no code is present, go to step 21).

15) Check HO2S (Engines With MAP Sensors)  Vacuum or air leaks in non-EEC-IV areas could cause Code 41, 91, 136, 172 or 176. Check following possibilities before continuing:

Turn ignition off. Ensure MAP sensor output voltage is correct. See CIRCUIT TEST DF, step 3). Disconnect appropriate HO2S from wiring harness connector. On 3-wire HO2S, connect DVOM to HO2S terminal at sensor wiring harness connector and negative battery terminal. On 4-wire HO2S, connect DVOM to HO2S and HO2S GND or SIG RTN terminal at sensor wiring harness connector. On all models, disconnect and plug vacuum line at MAP sensor. Start engine, and apply 10-14 in. Hg to MAP sensor. Run engine at 2000 RPM for 2 minutes. If reading is 0.5 volt or more, go to step 17). If reading is less than 0.5 volt, replace HO2S and repeat QUICK TEST.

16) Check HO2S (Engines With MAF Sensor)  Purpose of this test is to verify HO2S can generate a voltage signal of greater than 0.5 volt during KOER SELF-TEST. Vacuum or air leaks in non-EEC-IV areas could cause Code 41, 91, 136, 172 or 176. Check following possibilities before continuing:

Turn ignition off. Disconnect appropriate HO2S from harness. On 3-wire HO2S, connect DVOM to HO2S terminal at sensor wiring harness connector and negative battery terminal. On 4-wire HO2S, connect DVOM between HO2S terminal and HO2S GND or HO2S GND terminal at HO2S wiring harness connector. On all models, run engine at 2000 RPM for 2 minutes. Repeat KOER SELF-TEST and monitor HO2S voltage. If voltage is 0.5 volt or more at end of self-test, go to step 17). If voltage is less than 0.5 volt, replace HO2S and repeat QUICK TEST.

17) Check Continuity Of HO2S Ground Circuits  Turn ignition off. Install breakout box, leaving PCM disconnected. Disconnect suspect HO2S wiring harness connector. Inspect connector for damage, moisture and corrosion. Repair as necessary. Measure resistance between HO2S test pin at breakout box and HO2S terminal at sensor wiring harness connector. On 3-wire HO2S, measure resistance between HO2S GND test pin at breakout box and negative battery terminal. On 4-wire HO2S, measure resistance between SIG RTN test pin at breakout box and SIG RTN terminal at sensor wiring harness connector. Where applicable, measure resistance between HO2S GND and SIG RTN at breakout box. On all models, go to step 18) if each resistance is less than 5 ohms. If any resistance is 5 ohms or more, repair open circuit. Remove breakout box, and reconnect all components. Drive vehicle for 5 minutes at 55 MPH. Repeat QUICK TEST.

18) Check HO2S Circuit For Short To Ground  Turn ignition off. Leave breakout box installed and PCM disconnected. Disconnect HO2S. Measure resistance between HO2S circuit test pin and test pins No. 40, 46 and 49 at breakout box. If any reading is less than 10,000 ohms, repair short to ground. Drive vehicle at 55 MPH for 5 minutes and repeat QUICK TEST. If reading is 10,000 ohms or more, go to step 19).

19) Check HO2S For Short To Ground  Ensure ignition is off and PCM is disconnected. With HO2S disconnected, measure resistance between PWR GND and HO2S terminal at HO2S connector. On 4-wire HO2S, also measure resistance between HO2S GND and/or SIG RTN terminal at sensor wiring harness connector. If resistance is less than 10,000 ohms, replace HO2S. Drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST. If resistance is 10,000 ohms or more, perform following procedure as applicable.

20) Attempt To Eliminate Code 41/172, 91/136 Or 176 (Engines With MAP Sensor)  Turn ignition off. Connect PCM to breakout box. Connect HO2S. Disconnect and plug MAP sensor vacuum hose. Start engine and apply 10-14 in. Hg to MAP sensor. Increase engine speed to 2000 RPM for 2 minutes. Allow engine to return to idle. Perform KOER SELF-TEST. If diagnosing continuous memory codes, drive vehicle 5 miles at 55 MPH. If Code 41/172, 91/136 or 176 is still present, remove breakout box. Connect all components. If engine runs rough, go to CIRCUIT TEST S, step 2). If engine does not run rough, replace PCM, drive vehicle 5 miles at 55 MPH and repeat QUICK TEST. If code is not present, remove breakout box and connect all components. HO2S input circuit and fuel delivery are okay; fault is in area common to all cylinders. TESTS W/O CODES - EEC-IV article in this section.

21) Check Resistance Of HO2S Heating Element  Turn ignition off. Disconnect suspect HO2S. Inspect both ends of connector for damage, moisture and corrosion. Repair if necessary. Measure resistance between IGNITION RUN circuit and PWR GND circuit at HO2S connector. Hot to warm engine resistance should be 5-30 ohms. Cold engine resistance is 2-5 ohms. If resistance is within specification, go to step 22). If resistance is not as specified, replace HO2S and repeat QUICK TEST.

22) Check For Power At HO2S Harness Connector  Disconnect HO2S. Turn ignition on. Measure voltage between IGNITION RUN and PWR GND circuits at HO2S wiring harness connector. If voltage is 10.5 volts or more, HO2S system and fuel delivery are okay. HO2S may have cooled before KOER SELF-TEST. If driveability symptom continues, fault is in area common to all cylinders. TESTS W/O CODES - EEC-IV article in this section. If voltage is less than 10.5 volts, go to step 23).

23) Check Continuity Of Power Ground Circuit  Turn ignition off. Measure resistance between PWR GND circuit at HO2S wiring harness connector and negative battery terminal. If resistance is less than 5 ohms, repair open in IGNITION RUN circuit and repeat QUICK TEST. If resistance is 5 ohms or more, repair open in PWR GND circuit and repeat QUICK TEST.

24) Diagnostic Test Code 42/173, 92/137 Or 177: Check HO2S SIGNAL For Short To Power  On dual HO2S systems, Code 42/173 applies to right or rear sensor. Codes 92/137 and 177 applies to left or front sensor. HO2S continuously rich could be caused by:

Turn ignition off. Disconnect appropriate HO2S. Inspect connector for damage and repair as necessary. Turn ignition on. Measure voltage between HO2S and PWR GND terminal at HO2S wiring harness connector. If reading is less than 0.5 volt, go to step 26). If reading is 0.5 volt or more, go to step 25).

25) Check For Short To Power  Turn ignition off. Disconnect HO2S. Inspect wiring harness for damage and repair as necessary. Disconnect PCM 60-pin connector. Install Breakout Box (T83L-50-EEC-IV), leaving PCM disconnected. Disconnect suspect HO2S. Measure resistance between VPWR test pin and HO2S SIG test pin at breakout box. If reading is 10,000 ohms or more, replace PCM. Remove breakout box, and reconnect all components. Drive vehicle 5 miles at 55 MPH. Repeat QUICK TEST. If reading is less than 10,000 ohms, repair short circuit. Remove breakout box, and reconnect all components. Drive vehicle 5 miles at 55 MPH. Repeat QUICK TEST.

26) Check HO2S For Short To IGNITION RUN Circuit  Turn ignition off, and wait 10 seconds. Disconnect HO2S. Measure resistance between IGNITION RUN terminal and HO2S terminal at HO2S wiring harness connector. If resistance is 10,000 ohms or more, go to step 27) for Codes 42/173, 92/137 and 177 or step 28) (MAP sensor) or 30) (MAF sensor) for codes 171, 174, 175 and 178. If resistance is less than 10,000 ohms, replace HO2S. Drive vehicle for 5 miles at 55 MPH, and repeat QUICK TEST.

27) Attempt To Generate Code 41/172, 91/136 Or 176  Turn ignition off. Disconnect appropriate HO2S. Using jumper wire, connect HO2S terminal at HO2S wiring harness connector to negative battery terminal. Repeat KOER SELF-TEST. If Code 41/172, 91/136 or 176 is present, remove jumper wire and go to step 28) for engines with MAP sensor or step 30) for engines with MAF sensor. If Codes 41/172, 91/136 and 176 are not present, remove jumper wire. Disconnect PCM connector, and inspect it for damage. If connector is okay, replace PCM. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.



Because MAP sensor greatly influences fuel control, MAP sensor may malfunction without setting Code 22/126. Next 2 steps test MAP vacuum circuit.

28) Check MAP Sensor For Vacuum Leaks  Turn ignition off. Disconnect vacuum hose from MAP sensor. Inspect hose for damage and repair as necessary. Connect vacuum pump to sensor. Apply 18 in. Hg to MAP sensor. If sensor does not hold vacuum, replace MAP sensor. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST. If sensor holds vacuum, release vacuum and go to step 29).

29) Check For Loss Of Vacuum To MAP Sensor  Using vacuum "T", connect vacuum gauge in MAP sensor vacuum circuit. Start engine. Note vacuum reading after idle stabilizes. Turn ignition off. Remove "T" and vacuum gauge. Reconnect hose to MAP sensor. Connect vacuum gauge at a different manifold vacuum location. Start engine, and note vacuum reading. If readings do not differ by more than 1 in. Hg, go to next step. If readings differ by more than 1 in. Hg, inspect vacuum hoses for leaks, kinks and blockage. Repair as necessary. Connect all components, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

30) HO2S Check  Turn ignition off. Disconnect HO2S. For 4-wire HO2S, connect DVOM to HO2S terminal and HO2S GND or SIG RTN terminal at HO2S connector. For 3-wire HO2S, connect DVOM to HO2S terminal at HO2S connector and negative battery terminal. On all models, cause a vacuum leak to force HO2S to go lean. Remove PCV valve hose. Start engine. Raise engine speed to about 2000 RPM. DVOM should indicate less than 0.4 volt within 30 seconds. If voltage is not as specified, replace HO2S. Reconnect vacuum hoses, and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST. If voltage is as specified, go to step 90).

NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 30) to step 90). No test procedures have been omitted.

90) Check Continuous Monitor Mode  Start engine, and warm it to normal operating temperature. Raise engine speed to 2000 RPM for 2 minutes and then return to idle. Enter KOER wiggle test. See CONTINUOUS MONITOR MODE (WIGGLE TEST) under QUICK TEST. While observing DVOM or scan tester, shake or bend small sections of wiring harness from HO2S to PCM. Wiggle, shake or bend small sections of wiring harness from HO2S GND to PCM. If no fault is indicated, remain in CONTINUOUS MONITOR MODE and go to step 91). If fault is indicated, isolate and repair fault as necessary. Clear codes. Repeat QUICK TEST.

91) Continuous Monitor Test Drive Check  While still in KOER wiggle test, test drive vehicle for 5 miles at 55 MPH over smooth roads. Drive an additional 5 miles at 55 MPH over rough roads. If possible, drive vehicle through a pool of water to wet HO2S and connector. If fault is indicated, isolate fault and repair as necessary. Clear codes, and remove breakout box. Repeat QUICK TEST. If no codes are indicated, exit KOER wiggle test and go to next step.

92) Check HO2S Switching  Turn ignition off. Inspect wiring harness for proper routing and insulation. Check for burnt, chaffed or open wires. Repair if necessary. Disconnect PCM 60-pin connector, and inspect it for damaged or pushed-out pins, corrosion and other damage. Repair as necessary. Install breakout box. Connect PCM to breakout box. Connect analog voltmeter to suspect HO2S test pin and HO2S GND at breakout box. Test drive vehicle for 5 miles at 55 MPH while observing voltmeter. HO2S should switch from .3 volt to .9 volt within 3 seconds. If HO2S does not switch as described, replace HO2S. Connect all components, and repeat QUICK TEST. If HO2S switches as described, fault cannot be identified or duplicated at this time. See INTERMITTENTS in TESTS W/O CODES - EEC-IV article in this section.

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