Diagnostic Aids

Vacuum or air leaks in non-EEC-IV related areas may cause Code 41 or 91. Check the following areas: leaking vacuum actuator (A/C control motor, etc.) engine seals, EGR system, PCV system and lead contamination of HEGO sensor.

Fig 1: Checking HEGO Sensor

16) Check HEGO Sensor On Engines With MAF Sensor The purpose of this test is to verify HEGO sensor can generate a voltage signal of greater than 0.5 volt during KOER SELF-TEST. With ignition off and DVOM set on 20-volt scale, disconnect appropriate HEGO sensor from harness. Depending on type of sensor (3 wires or 4 wires), connect DVOM as follows:

4-Wire HEGO Sensors - Connect DVOM between HEGO SIGNAL and HEGO GND at HEGO sensor connector.
3-Wire HEGO Sensors - Connect DVOM to HEGO SIGNAL at sensor and battery negative terminal.

Rerun KOER SELF-TEST and monitor HEGO sensor voltage. If voltage is greater than 0.5 volt, go to next step. If voltage is less than 0.5 volt, replace HEGO sensor and rerun QUICK TEST.

Fig 1: Checking HEGO Sensor

17) Check Continuity Of HEGO SIGNAL & HEGO GROUND Circuits Turn ignition off. Install breakout box, leaving ECA disconnected. Disconnect suspected HEGO sensor from vehicle harness. Inspect both ends of connector for damage or pushed-out pins, moisture, corrosion, loose wires or other damage. Repair as necessary. Set DVOM on 200-ohm scale. Measure resistance between HEGO SIGNAL (test pin No. 29) at breakout box and HEGO SIGNAL at vehicle harness connector.

On 3-Wire HEGO sensors, measure resistance between HEGO GROUND test pin at breakout box and battery negative terminal.

On 4-Wire HEGO sensors, measure resistance between HEGO GROUND test pin at breakout box and HEGO GND at vehicle harness connector. Where applicable, measure resistance between HEGO GND and SIG RTN at breakout box.

If each resistance is less than 5 ohms, go to next step. If each resistance is greater than 5 ohms, repair open circuit. Remove breakout box and reconnect all components. Drive vehicle for 5 minutes at 55 MPH. Rerun QUICK TEST.

18) Check HEGO Circuit For Short To Ground Turn ignition off. Leave breakout box installed and ECA disconnected. Disconnect HEGO sensor and set DVOM on 200-k/ohm scale. Measure resistance between HEGO SIGNAL (test pin No. 29) and test pins No. 40, 46 and 49 at breakout box. If any reading is less than 10 k/ohms, repair short to ground; remove breakout box, reconnect all components and repeat QUICK TEST. If reading is 10 k/ohms or more, go to next step.

19) Check HEGO Sensor For Short To Ground With ignition off, breakout box installed and ECA disconnected, set DVOM to 200-k/ohm scale. Disconnect HEGO sensor. Measure resistance between PWR GND and HEGO SIGNAL at HEGO sensor connector. For 3.8L and trucks with 4-wire sensors, measure resistance between PWR GND, HEGO GND, SIG RTN and HEGO SIG at HEGO sensor connector. If resistance is less than 10 k/ohms, replace HEGO sensor and remove breakout box; reconnect all components, drive vehicle for 5 miles at 55 MPH and repeat QUICK TEST. If resistance is greater than 10 k/ohms, perform the following procedure as applicable.

For codes 144/41, 139/91, 171, 174, 175 or 178, go to step 90).
On engines with MAP sensor, go to step 20).
On engines with MAF sensor, remove breakout box. Reconnect HEGO sensor and replace ECA. Drive vehicle for 5 miles at 55 MPH. Rerun QUICK TEST.

20) Attempt To Eliminate Code 41/172, 91/136 Or 176: Engines With MAP Sensor Turn ignition off. Install breakout box; reconnect ECA and HEGO sensor. Ensure MAP sensor vacuum hose is disconnected and plugged. Apply 10-14 in. Hg to MAP sensor. Start and run engine at 2000 RPM for 2 minutes. Allow engine to return to idle. Perform KOER SELF-TEST. If you are here for Continuous Memory Codes, vehicle has to be driven 5 miles at 55 MPH. If Code 41/172, 91/136 or 176 is still present, remove breakout box. Reconnect ECA and MAP sensor vacuum line. If engine still runs rough, go to TEST S, step 2). If engine does not run rough, replace ECA; drive vehicle 5 miles at 55 MPH then repeat QUICK TEST.

If Code 41/172, 91/136 or 176 is not present, remove breakout box and reconnect ECA and MAP sensor vacuum hose. HEGO input circuit and fuel delivery are okay; fault is in area common to all areas. Service as necessary.

21) Check Resistance Of HEGO Heating Element Turn ignition off. Disconnect suspected HEGO sensor. Inspect both ends of connector for damaged or pushed-out pins, moisture, corrosion, loose wires or other damage. Repair as necessary. Set DVOM on 200-ohm scale. Measure resistance between KPWR circuit and PWR GND circuit at HEGO sensor connector. Hot to warm resistance should be 5.0-30.0 ohms. Room temperature resistance is 2.0-5.0 ohms. If resistance is within specification, go to next step. If resistance is not as specified, replace HEGO sensor and repeat QUICK TEST.

Fig 2: Checking HEGO Sensor

22) Check For Power At HEGO Harness Connector With ignition on and engine off, and HEGO disconnected, set DVOM on 20-volt scale. Measure voltage between KEY POWER circuit and PWR GND circuit at HEGO vehicle harness connector. If voltage is greater than 10.5 volts, HEGO system and fuel delivery are okay; HEGO sensor may have cooled before KOER SELF-TEST. If driveability symptom persists, fault is in area common to all cylinders, such as air or vacuum leaks, fuel contamination, EGR, MAP, etc. If voltage is less than 10.5 volts, go to next step.

Fig 3: Identifying HEGO Sensor Harness Connector

23) Check Continuity Of Power Ground Circuit Turn ignition off and wait 10 seconds. Disconnect HEGO sensor and set DVOM on 200-ohm scale. Measure resistance between PWR GND 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 and rerun QUICK TEST. If resistance is more than 5 ohms, reconnect HEGO sensor and repair open in PWR GND circuit. Reconnect HEGO sensor and rerun QUICK TEST.

24) Check HEGO SIGNAL For Short To Power On dual HEGO systems, Code 42/173 refers to right (or rear) side HEGO, and Code 92/137 or 177 refers to left (or front) side HEGO. Turn ignition off and wait 10 seconds. Disconnect appropriate HEGO sensor. Set DVOM on 20-volt scale. With ignition on and engine off, measure voltage between HEGO SIG and PWR GND at HEGO vehicle harness connector. If reading is less than .5 volt, go to step 26). If reading is NOT less than 0.5 volt, go to next step.

Fig 4: Identifying HEGO Sensor Harness Connector

25) Check For Short To Power Turn ignition off. Disconnect HEGO sensor. Inspect both ends of connector for damaged or pushed-out pins, moisture, corrosion, loose wires or other damage. Repair as necessary. Set DVOM on 200-k/ohm scale. Disconnect ECA 60-pin connector. Install breakout box, leaving ECA disconnected. Measure resistance between KEY POWER circuit and HEGO SIG circuit at breakout box.

If reading is greater than 10 k/ohms, replace ECA. Remove breakout box and reconnect all components. Drive vehicle for 5 miles at 55 MPH. Rerun QUICK TEST.

If reading is less than 10 k/ohms, repair short circuit. Remove breakout box and reconnect all components. Drive vehicle for 5 miles at 55 MPH. Rerun QUICK TEST.

26) Check HEGO Sensor For Short To Ignition Run Circuit Turn ignition off and wait 10 seconds. Disconnect HEGO. Set DVOM to 200-k/ohm scale. Measure resistance between KEY POWER circuit and HEGO SIG circuit at HEGO sensor connector.

If resistance is greater than 10 k/ohms, go to next step for codes 42/173, 92/137 or 177. For codes 171, 174, 175 or 178, go to step 28) (MAP sensor), or step 30) (MAF sensor).

If resistance is less than 10 k/ohms, replace HEGO sensor. Drive vehicle for 5 miles at 55 MPH and rerun QUICK TEST.

Fig 5: Checking HEGO Sensor

27) Attempt To Generate Code 41/172, 91/136 Or 176 The following non-EEC areas may cause these codes: fuel-contaminated engine oil, ignition misfire and faulty CANP system. Turn ignition off and wait 10 seconds. Disconnect appropriate HEGO sensor. Using jumper wire, connect HEGO SIG circuit at HEGO vehicle harness connector to battery negative terminal. Repeat KOER SELF-TEST.

If Code 41/172, 91/136 or 176 is present, remove jumper wire and go to next step for engines with MAP. For all other engines, go to step 30).

If Codes 41/172, 91/136 or 176 is not present, remove jumper wire. Disconnect ECA connector and inspect for damage. If connector is okay, replace ECA. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

Because MAP sensor has a large influence on fuel control, Code 42/172 or 91/136 may be caused by a faulty MAP sensor, even though Code 22/126 has not been set. The next 2 steps test the MAP vacuum circuit.

28) Check MAP Sensor For Vacuum Leaks Turn ignition off and wait 10 seconds. Disconnect vacuum hose from MAP sensor and connect vacuum pump to sensor. Apply 18 in Hg to MAP sensor. If sensor holds vacuum, release vacuum and go to next step. If sensor does not hold vacuum, replace MAP sensor. Remove vacuum pump. Connect vacuum hose and reconnect HEGO. Drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST.

29) Check For Loss Of Vacuum To MAP Sensor Using vacuum "T", connect vacuum gauge in intake manifold vacuum hose at MAP sensor. Start engine and note vacuum reading at a stable idle. Turn ignition off and wait 10 seconds. Remove "T" and vacuum gauge. Reconnect hose to MAP sensor. Connect vacuum gauge at a different intake manifold 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, or blockage. Repair as necessary. Reconnect all components and drive vehicle for 5 miles at 55 MPH. Rerun QUICK TEST.

30) HEGO Sensor Check Turn ignition off and wait 10 seconds. Disconnect HEGO sensor. Set DVOM on 20-volt scale. For 4-Wire HEGO, connect DVOM to HEGO SIGNAL and HEGO GND at HEGO sensor connector. For 3-Wire HEGO, connect DVOM to HEGO SIGNAL at HEGO sensor connector and battery negative post. Remove a vacuum hose to create vacuum leak, which will cause HEGO sensor to go lean. Start engine and run at approximately 2000 RPM. DVOM should indicate less than 0.4 volt within 30 seconds. If voltage is not as specified, replace HEGO sensor. Reconnect vacuum hoses and drive vehicle for 5 miles at 55 MPH. Repeat QUICK TEST. If voltage is as specified, go to next step.

Fig 1: Checking HEGO Sensor

NOTE: There is a break in the step numbering sequence at this point, skipping from step 30) to step 90). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.

90) Check Continuous Monitor Mode Turn ignition off and wait 10 seconds. Ensure engine is at normal operating temperature. Start engine and run at 2000 RPM for 2 minutes then return to idle. Enter KOER wiggle test. See CONTINUOUS MONITOR MODE (WIGGLE TEST) under QUICK TEST. While observing DVOM or "Scan" Tool, wiggle, shake or bend small sections of EEC-IV wiring harness from HEGO sensor to ECA. Wiggle, shake or bend small sections of EEC-IV wiring harness from HEGO GND to ECA. If no fault is indicated, remain in CONTINUOUS MONITOR MODE and go to next step. If fault is indicated, isolate and repair fault as necessary. Clear codes, remove breakout box and reconnect all components. Rerun QUICK TEST.

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

92) Check HEGO Switching Turn ignition off and wait 10 seconds. Inspect EEC-IV wire harness for proper routing and insulation. Check for burnt, chaffed or intermittently shorted or open wires. Repair as necessary. Disconnect ECA 60-pin connector and inspect for damaged or pushed-out pins, corrosion, loose wires or other damage; repair as necessary. Install breakout box and connect ECA to breakout box. Connect analog voltmeter to suspected HEGO sensor test pin and HEGO GND at breakout box. Test drive vehicle for 5 miles at 55 MPH while observing voltmeter. HEGO should switch from .3 volt to .9 volt within 3 seconds. If HEGO does not switch as described, replace HEGO sensor. Remove breakout box, reconnect all components and rerun QUICK TEST. If HEGO switches as described, fault cannot be identified or duplicated at this time. See INTERMITTENTS in the TESTS W/O CODES article in this section.