Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Diagnostic Aids
Perform this test when instructed during QUICK TEST or if directed by other test procedures. See HOW TO USE CIRCUIT TESTS under CIRCUIT TESTS before performing this test. 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:
- HEGO sensor.
- HEGO signal, ground circuit and sensor connection.
- Vacuum systems.
- Fuel injector and/or fuel injector circuitry.
- ECA.
- Electrical circuits (HEGO GND, HEGO, INJ 1-8, VPWR and SIG RTN).
To prevent replacement of good components, be aware following non-EEC areas may be cause of driveability concerns:
- Ignition system.
- Faulty evaporative emission system.
- EGR system.
- Air intake system.
- Engine oil contamination.
- Fuel system.
- Intake or exhaust system leaks.
- Engine cooling system.
| Service Code | HEGO Sensor Orientation | Fault Definition |
|---|---|---|
| 41/172r 91/136r 176c | Right or Rear Left or Front Left or Front | HEGO Lean, Lack Of Switching |
| 42/173r 92/137r 177c | Right or Rear Left or Front Left or Front | HEGO Rich, Lack Of Switching |
| 144/41c 139/91c | Right or Rear Left or Front | No HEGO Switch Detected |
| 171c 175c | Right or Rear Left or Front | Adaptive Fuel Limit Reached, Lack Of HEGO Switching |
| 174c 178c | Right or Rear Left or Front | Lazy HEGO, Slow To Switch |
| Engine | Ohms |
|---|---|
| 2.3L OHC & 3.0L | 15.0-18.0 |
| All Other Engines | 13.0-16.0 |
| Engine | Ohms |
|---|---|
| 2.3L HSC | 6.0-8.0 |
| 2.3L OHC | 7.0-9.5 |
| 2.9L, 3.0L & 4.9L | 5.0-6.5 |
| 5.0L, 5.8L | 3.0-4.0 |
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.