logic check - obd code 21 (1 Viewer)

[CharlieS]

I'm looking for a quick check on my troubleshooting logic.

I have a 94 with new denso o2 sensors that recently started to throw a persistent obd code 21 - heated o2 sensor on cyl 1,2,3.

I swapped the two sensors, and expected that if the failure was on the o2 sensor side, to code would flip to a 28 (issue with bank 4,5,7 heated o2). The code remained 21.

This leads me to believe I have a vehicle harness side issue on the harness to the cyl 1, 2, 3 o2 sensor.

I tested both harnesses at startup and both are getting ~14v at the yellow wires.

This leads me toward the ground circuit.

My next step is to check for continuity on HT1 and HT2 from the ECM to the harnesses at the sensors. Grey wire (HT1) and orange wire (HT2) on pin 1 of each o2 connector.

Does this seem like the right approach?

 

[Malleus]

That's what I'd do, if I didn't have the FSM. I haven't checked to see what the '94 troubleshooting plan is.

 

[Buckums]

Maybe this will help..

 

[CharlieS]

Thank you both.

...And today, the check engine light and code is gone. I didn't do anything other than remove and clean the terminals with Deoxit and plug them back in.

I must have a loose connection or broken wire, but boy oh boy, I hate diagnosing intermittent faults.