The vehicle is a 1998 GMC Jimmy 4-door 4-wheel drive. I am using an autoengineuity scanner.

Oxygen sensor B1S1 shows 447mv at key-on engine-off like the other two sensors. Within seconds after start the car it drops to 0 mv. If it were just a bad sensor I'd expect it to stay at zero.

I am planning to trace the wiring harness and it would be helpful if someone could tell me hoe the 02 sensor on the drivers side is routed. I am guessing this is a short to ground when the insulation heats up on a wire.

I may also swap two of the sensors to see if the problem moves with the sensor or not.

I am also open to other ideas on how to diagnose this sensor.

 

Is the SES light on? What DTC's are in memory?

 

No DTC - SES is off. Will not go into closed loop. The 02 sensor monitoring page says it is in open loop due to a fault condition. I checked the O2 sesnor readouts and say that B1S1 was reading zero. Then I noticed the Key-On setting and the value dropping after starting the engine.

 

Does the SES light come on, and then turn off, with the bulb check when you initially turn the key to the ON position? Are there any DTC's in the pending file?

 

SES light works - it comes on with Key-ON. No current or pending DTC's. I have an O2 sensor B1S1 that reports 0mv once the vehicle is started. Scan tool reports that system is not in closed loop due to a fault.

 

The O2 sensor is not causing the PCM to stay in open loop. There are certain parameters and requirements that must be met before the PCM switches to closed loop operation. Keep in mind that the PCM ignores oxygen sensors on start up. The PCM determines when to go into closed loop, not the oxygen sensors. Perform a drive cycle General Motors Driving Cycle This will allow all of the monitors to run and complete their tests. After completing the drive cycle, check I/M Readiness and make sure all monitors have run and completed. If there are any failures, a DTC will be set. The PCM is an invaluable piece of test equipment, use it, and trust it. PCM's are darn near bullet-proof and rarely fail. If there are no DTC's in memory, or pending, the system is working and you can rest assured there are no problems with it.

 

Drove the drive cycle and got the code to set P0154 to set. Following the diagnostics step 8 tells me to check resistance between Ho2s LOW circuit and VCM ground circuit. Step 2 read 447mv and step 4 read 0-4mv (alternating). Step 7 lit the test lamp.

HO2S Low circuit is pin 21 on blue ecm connector. Where is a ground at a VCM connector?

When testing resistance to ground using battery ground I have an open circuit.

Working on a 1998 GMC Jimmy

 

P0154 "Insufficient activity Bank 2 sensor 1". This DTC refers to the oxygen sensor closest to the exhaust manifold on the passenger side.

Not sure what flow chart you're using... on 1998 the LOW circuit is the tan wire, pin #7 in the blue connector. The HIGH circuit is the purple wire, pin #21 in the blue connector.

VCM ground is in the "clear" connector, pins #17 & #18. Both are black with white trace wires. They end up splicing together and grounding on the rear of the left cylinder head.

EDIT:
Step 8 wants you to check resistance from pin #7 in the blue connector to pins #17 and #18 in the "clear" connector with both VCM connectors disconnected from the VCM.

 

You are correct - pin #7. Open circuit. Ground checks out okay. No resistance to block or battery terminal. Continuity of tan wire from o2 sensor connector to VCM connector is 0.1 ohms. Wiring looks good. Must be an open sensor/sensor harness requiring a new sensor. Do you agree?

 

I agree, the LOW circuit for the HO2 sensor has high resistance, (open), should be 500 ohms or less. This could be one of two things: either an ungrounded exhaust system, (not likely) or a faulty sensor. Stick a sensor in there, clear the memory and perform another drive cycle to confirm the repair.

EDIT:
You probably only need to do step 4 of the drive cycle if all of the monitors completed during the first drive cycle.