* Wiring Harness Issues: This is the most likely culprit after replacing the sensor. Check the wiring between the O2 sensor and the ECM (Engine Control Module) for:
* Broken wires: Look for cuts, chafing, or any damage to the insulation.
* Loose connections: Ensure all connectors are securely plugged in at both the sensor and the ECM. Corrosion can also be a problem, so clean the connections carefully.
* Shorts to ground or power: A short circuit can send the wrong signal to the ECM. This requires careful testing with a multimeter to identify.
* High resistance in the wiring: A high resistance in the wiring can prevent the proper signal from reaching the ECM. Again, a multimeter is needed to test resistance.
* ECM (Engine Control Module) Issues (Despite Replacement): While you've ruled this out, it's worth considering if the replacement ECM is faulty or incorrectly programmed. This is less likely but possible. Getting a professional diagnosis from an auto electrician would be helpful in this scenario.
* Ground Problems: A poor ground connection for the O2 sensor circuit can cause erratic readings and set the code. Check the ground points associated with the sensor and the ECM.
* Heater Circuit (If Applicable): Some O2 sensors have a heater element. If the heater circuit is faulty, it could affect the sensor's readings and trigger the code. This requires further investigation of the specific sensor and wiring diagram.
Troubleshooting Steps:
1. Visual Inspection: Thoroughly examine the O2 sensor wiring harness for any obvious damage.
2. Connector Inspection: Carefully inspect all connectors for corrosion or loose connections. Clean them with electrical contact cleaner.
3. Wiring Continuity Test: Use a multimeter to test the continuity of the O2 sensor wires from the sensor to the ECM. Refer to a wiring diagram for your specific truck model to identify the correct wires.
4. Voltage/Signal Check: Use a multimeter to check the voltage and signal at the O2 sensor connector while the engine is running. Compare these readings to the specifications in a repair manual.
5. Ground Check: Check for a good ground connection at the O2 sensor and the ECM.
Important Note: You'll need a wiring diagram for your specific 1988 Chevy truck to accurately test the O2 sensor circuit. These diagrams are often available online through repair manual websites or forums dedicated to classic Chevy trucks. If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic for diagnosis and repair.
