LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Ford >> 1993 >> Crown Victoria Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Tests W/Codes - EEC-IV >> Circuit Tests >> Circuit Test H - Fuel Control >> SFI
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Circuit Test H - Fuel Control: SFI

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 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 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.

scientia non olet · About LEMON Manuals