Diagnostic Aids

Perform this test when directed by QUICK TEST. Fuel-contaminated engine oil may affect Codes 41/172 or 42/173. If this is suspected, remove PCV valve from valve cover and rerun QUICK TEST . If problem is corrected, change engine oil and filter.

This test is intended to diagnose ECA, HEGO sensor, HEGO signal and ground circuits, HEGO sensor connection, vacuum systems, fuel injectors and following harness circuits.

HEGO GROUND
HEGO INJ 1-6
SIG RTN
VPWR

Fig 1: Fuel Control Circuits

To prevent replacement of good components, be aware that following non-EEC related areas and components may be cause of problem.

Air Filter
CANP Problems
Distributorless Ignition System
Engine Less Than Normal Operating Temperature
Fouled Spark Plugs
Fuel Or Engine Oil Contamination
Ignition Coil
Intake Or Exhaust Manifold Leaks
PCV Valves
Poor Power Ground
Spark Plug Wires.

INJECTOR RESISTANCE

Application

Ohms

Single Injector ⁽¹⁾

13.0-16.0

Injector Bank

4.0-5.5

⁽¹⁾	Resistance values are for a single injector.

FUEL PRESSURE SPECIFICATIONS

Application	psi (kg/cm² )
KOEO Pressure	35-45 (2.46-3.16)
KOER Pressure	30-45 (2.11-3.16)

1) Check Fuel Pressure - Code 41/172 indicates system is lean and may indicate an exhaust odor/smoke condition. Loose HEGO Signal or HEGO ground connection or poor HEGO harness insulation near exhaust manifold will cause an intermittent fault causing ECA to determine a false lean condition to be present. Also check other non-EEC related areas and components that may be cause of problem. Code 42/173 indicates system is rich. Turn ignition off. Wait 10 seconds. Install fuel pressure gauge. Connect manifold vacuum to fuel pressure regulator (if applicable). Start engine, and operate it at idle. If engine will not start, cycle ignition off and on several times. Maximum fuel pressure is obtained at WOT or with vacuum hose disconnected from fuel pressure regulator. Check if fuel pressure is within specification. If fuel pressure is within specification, go to next step. If fuel pressure is not within specification, see FUEL SYSTEM in I - SYS/COMP TESTS - FUEL INJECTION article.

2) Check Ability Of System To Hold Fuel Pressure - Turn ignition on, leaving engine off. If fuel pressure remains at specification for 60 seconds and engine does not start, go to next step. If fuel pressure remains at specification for 60 seconds and engine runs rough and/or misses or Code 41, 42, 172 or 173 is present, remove fuel pressure gauge and go to step 4). If fuel pressure does not remain at specification for 60 seconds, go to step 6).

3) Fuel Delivery Test - Ensure fuel is good quality. Turn ignition off. Install fuel pressure gauge. Pressurize fuel system as in step 1). Disconnect inertia switch. Crank engine for 5 seconds. If fuel pressure drop is more than 5 psi after 5 seconds of cranking, EEC-IV system is not cause of no-start condition. Remove fuel pressure gauge, reconnect inertia switch and check additional no-start tests in TESTS W/O CODES article. If fuel pressure drop is less than 5 psi after 5 seconds of cranking, remove fuel pressure gauge, reconnect inertia switch and go to next step.

4) Check Injector & Harness Resistance - Turn ignition off. Wait 10 seconds. Disconnect and inspect ECA 60-pin connector. With breakout box installed, disconnect ECA. Set DVOM on 200-ohm scale. Measure resistance of injector banks between test pins No. 37 and 58 (injector bank No. 1) or 37 and 59 (injector bank No. 2) at breakout box. See INJECTOR RESISTANCE table. If all readings are within specification, go to step 6). If any reading is not within specification, repair open in VPWR circuit (for no start) or go to next step.

5) Isolate Faulty Injector Circuit - Turn ignition off. With breakout box installed, disconnect ECA. Disconnect all injectors on suspected bank. Set DVOM on 200-ohm scale. Connect one injector and measure resistance between test pins No. 37 and 58 or 59 as required. Disconnect injector and repeat process for all other injectors. If all resistance readings are 13-16 ohms, go to next step. If any resistance reading is less than 13 ohms or greater than 16 ohms, repair open or short in VPWR circuit or in injector circuit of suspected injector. If circuits are okay, replace suspected injector. Remove breakout box, reconnect ECA and injectors. Drive vehicle for 5 miles at 55 MPH, and then rerun QUICK TEST .

6) Check Injector Drive Signal - Turn ignition off. With breakout box installed, connect ECA to breakout box. Connect a non-powered 12-volt test light between test pins No. 37 and 59 at breakout box. Crank or start engine. If test light glows dimly, system is operating properly. Go to next step. If test light does not light, check if 12 volts are present at test pins No. 37 and 57. If test light glows brightly, check for short to ground in injector circuit. If circuit is okay, replace ECA. Remove breakout box, and rerun QUICK TEST .

7) Check External Source For Fuel Pressure Problem - Turn ignition off. Pressurize fuel system as in step 1). Visually inspect fuel injector "O" rings, fuel pressure regulator and fuel rails for fuel leakage. If no visible leakage is found, go to next step. If fuel leakage is visible, remove fuel pressure regulator, repair leak as necessary and rerun QUICK TEST .

8) Injector Balance Test - Connect tachometer to engine. Operate engine at idle. Disconnect and then reconnect fuel injectors individually. Idle Speed Control (ISC) will try to reestablish RPM. If all injectors produce a momentary drop in RPM of at least 100 RPM, fuel system is okay. Check fuel system for other possible causes of problem. If any injectors do not produce a momentary drop in RPM of at least 100 RPM, see FUEL CONTROL in I - SYS/COMP TESTS - FUEL INJECTION article. Clean injectors, and rerun QUICK TEST .

NOTE: A break in step numbering sequence occurs at this point. Test skips from step 8) to step 11). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

11) Code 41/172: Fuel Control Always Lean - Code 41/172 indicates system is lean. Run engine at 2000 RPM for 2 minutes. Turn ignition off, and wait 10 seconds. Rerun KOER SELF-TEST. If Code 41 or 172 is present, go to step 14). If Code 41 or 172 is not present, go to step 20).

NOTE: A break in step numbering sequence occurs at this point. Test skips from step 11) to step 14). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

14) Check HEGO Sensor - Purpose of this test is to verify HEGO sensor can generate a voltage signal of greater than .5 volt during KOER SELF-TEST. Vacuum or air leaks in non-EEC-IV related areas may cause Code 41/172 or 91/136. Following are possible causes for these faults.

Engine Seals
Leaking Vacuum Actuator (A/C Control Motor, Etc.)
PCV System
Unmetered Air Leaks Between MAF Sensor And Throttle Body
Lead-Contaminated HEGO Sensor

Turn ignition off. Disconnect HEGO sensor from vehicle harness. Connect DVOM between HEGO SIGNAL and HEGO GROUND at HEGO sensor connector. See Fig 2 . Set DVOM on 20-volt scale. Run engine at 2000 RPM for 2 minutes. Rerun KOER SELF-TEST, and monitor HEGO sensor voltage. If voltage is greater than .5 volt, go to next step. If voltage is less than .5 volt, replace HEGO sensor and rerun QUICK TEST.

Fig 2: Identifying HEGO Sensor Terminals

15) Check Continuity Of HEGO SIGNAL & HEGO GROUND Circuits - Turn ignition off. Disconnect and inspect ECA 60-pin connector. With breakout box installed, disconnect ECA. Disconnect HEGO sensor. Set DVOM on 200-ohm scale. Measure resistance between test pins No. 46 and 49 at breakout box and HEGO GROUND at vehicle harness connector. See Fig 3 . If all readings are less than 5 ohms, go to next step. If any reading is greater than 5 ohms, repair open in circuit. Remove breakout box, and reconnect ECA, HEGO sensor and all other disconnected or removed components. Rerun QUICK TEST.

Fig 3: Identifying HEGO Sensor Vehicle Harness Terminals

16) Check HEGO Circuit For Short To Ground - Turn ignition off. With breakout box installed, disconnect ECA. Disconnect HEGO sensor. Set DVOM on 200-k/ohm scale. Measure resistance between test pin No. 29 and test pins No. 40, 46 and 49 at breakout box. If all resistance readings are greater than 10 k/ohms, go to next step. If any resistance reading is less than 10 k/ohms, repair short in circuit. Reconnect ECU and HEGO sensor. Rerun QUICK TEST.

17) Turn ignition off. With breakout box installed, disconnect ECA. Disconnect HEGO sensor. Set DVOM on 200-k/ohm scale. Measure resistance between PWR GROUND and HEGO SIGNAL at HEGO sensor connector. Measure resistance between PWR GROUND and SIG RTN at HEGO sensor connector. If all readings are less than 10 k/ohms, replace HEGO sensor. Reconnect ECA, and rerun QUICK TEST . If any reading is greater than 10 k/ohms, replace ECA. Remove breakout box, reconnect HEGO sensor and rerun QUICK TEST .

NOTE: A break in step numbering sequence occurs at this point. Test skips from step 17) to step 20). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

20) Check Resistance Of HEGO Heating Element - Turn ignition off. Disconnect HEGO sensor. Set DVOM on 200-ohm scale. Measure resistance between KPWR and PWR GND circuits at HEGO sensor connector. Hot to warm resistance should be 5-30 ohms. Room temperature resistance is 2-5 ohms. If resistance is within specification, go to next step. If resistance is not within specification, replace HEGO sensor and rerun QUICK TEST .

21) Check For Power At HEGO Harness Connector - Turn ignition on, leaving engine off. Disconnect HEGO sensor. Set DVOM on 20-volt scale. Measure voltage between KEY POWER and PWR GROUND circuits at HEGO vehicle harness connector. If voltage is less than 10.5 volts, go to next step. If voltage is greater than 10.5 volts, HEGO sensor system is okay. Reconnect HEGO sensor, and go to step 1).

22) Check Continuity Of Power Ground Circuit - Turn ignition off, and wait 10 seconds. Disconnect HEGO sensor. Set DVOM on 200-ohm scale. Measure resistance between PWR GROUND circuit at HEGO vehicle harness connector and battery negative post. If resistance is less than 5 ohms, repair open in KEY POWER circuit. Reconnect HEGO sensor, and rerun QUICK TEST. If resistance is greater than 5 ohms, repair open in PWR GROUND circuit. Reconnect HEGO sensor, and rerun QUICK TEST.

23) Code 42/173, 92/137 Or 177: Fuel Control Always Rich; Check HEGO SIGNAL For Short To Power - With dual HEGO sensors, Code 42/173 refers to right or rear HEGO sensor. Code 92/173 or 177 refers to left or front HEGO sensor. Code 42/173 indicates system is rich. Turn ignition off, and wait 10 seconds. Disconnect appropriate HEGO sensor for code present. Set DVOM on 20-volt scale. Turn ignition on, leaving engine off. Measure voltage between HEGO SIGNAL and PWR GROUND circuits at HEGO vehicle harness connector. If voltage is less than .5 volt, go to next step. If voltage is greater than .5 volt, repair short to power in HEGO sensor circuit. Reconnect HEGO sensor, and rerun QUICK TEST .

24) Check HEGO Sensor For Short To Ignition Run Circuit - Turn ignition off. Disconnect HEGO sensor. Set DVOM on 200-k/ohm scale. Measure resistance between KEY POWER and HEGO SIGNAL circuits at HEGO sensor connector. If resistance is greater than 10 k/ohms, go to next step. If resistance is less than 10 k/ohms, replace HEGO sensor and rerun QUICK TEST .

25) Try To Generate Code 41/172, 91/136 Or 176 - Fuel- contaminated engine oil, ignition misfire (fouled spark plug) and CANP problems may cause these codes. Turn ignition off. Wait 10 seconds. Disconnect HEGO sensor. Using jumper wire, connect HEGO SIGNAL circuit at HEGO vehicle harness connector to negative battery post. Rerun KOER SELF-TEST. If Code 41, 91, 136, 172 or 176 is present, remove jumper wire and go to step 28). If Codes 41, 91, 136, 172 and 176 are not present, remove jumper wire and reconnect HEGO sensor. Disconnect and inspect ECA 60-pin connector. Repair as necessary. If connector is okay, replace ECA and rerun QUICK TEST .

NOTE: A break in step numbering sequence occurs at this point. Test skips from step 25) to step 28). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

28) HEGO Sensor Check - Turn ignition off, and wait 10 seconds. Disconnect HEGO sensor. Connect DVOM to HEGO SIGNAL at HEGO sensor connector and to battery negative post. Set DVOM on 20-volt scale. Remove vacuum hose to create vacuum leak, which will cause HEGO sensor to go lean. Disconnect any vacuum hose from manifold vacuum tree. Disconnect PCV valve hose from PCV valve. Start engine, and run it at about 2000 RPM. DVOM should indicate less than .4 volt within 30 seconds. If voltage is not as specified, replace HEGO sensor. Reconnect vacuum hoses, and rerun QUICK TEST . If voltage is as specified, HEGO sensor is okay. Reconnect HEGO sensor and vacuum hoses, and go to step 1).