Upstream Oxygen Sensors - On with open loop tune?

Tripoli · Jan 24, 2010

A couple years back I started having problems with my upstream oxygen sensors. The circuit they were on kept blowing its 20A fuse under the dash. Sometimes this wouldn't happen for a few days, and sometimes it would happen as soon as I turned the key to ON before even starting the car, but usually it happened about 2 minutes into my drive.

This was a few months before I was going to put new heads on the car, so I wanted to try to get it fixed before I got my tune. Unplugging the other devices on the circuit had no effect. Unplugging the O2s always made it stop. I spent days going over it, trying different things and never figured out what was going on. Eventually, I gave up, and when I got my heads, I had the tuner do an open loop tune, where the tune does not use the oxygen sensors.

Today, I think I may have stumbled across the problem. I found a piece of exposed wire in the O2 extensions which came with my headers. I replaced the wire and plugged the upstream O2s back in. I started the car and let it run for about 3 minutes, then turned it off and repeated this a few times. Fuse didn't blow. Then I drove around for about 20 minutes until she was up to full operating temperature. Fuse is still good.

My question is: With an open loop tune, do the upstream oxygen sensors get power, or could the tune actually prevent them from powering up at all? I plan on getting back under there next week to stick a test light in there to see if it does anything, but I ran out of time tonight. I really want to understand how the circuit works. My baby's getting a D-1SC in a few weeks and I need to know my O2s are working properly by then.

wmburns · Jan 24, 2010

The engine VPWR buss is on all of the time that the key is on.

The PCM can control the O2 heaters via a ground start (PCM supplies ground to complete the circuit).

So let's assume a wiring fault lies after the O2 heater, then it wouldn't matter when the PCM called for O2 heat.

If the fault lies ahead of the O2 sensors, then the VPWR buss is active all the time. There is power available to cause the fuses to blow.

The only why the PCM could affect the fuse blowing would be if the O2 heater was drawing too much power. The the fuse would blow when the PCM called for O2 heat.

So the answer to your question is yes and Yes (qualified). The open loop tune did not prevent the fuse from blowing. Disconnecting them did.

Tripoli · Feb 14, 2010

So, the fuse still hasn't blown, and I'm still on an open loop tune. We unplugged the sensor which we believed has been the problem and we used a test light to see which of the 4 leads were getting power. Only one seemed to get power, even after the car had been running for about 3 minutes. Should two of the wires be powered, or just the one?

wmburns · Feb 14, 2010

As stated before, only ONE wire (RD/YE) will have +12 volt power all of the time. This is true regardless of the operational mode of the PCM. Even if "open loop" is forced on via PCM programming, the O2 RD/YE wire will have +12 volts all of the time the key is in the on position.

The heated O2 sensors are activated by the PCM grounding the return leads.

If you detect any voltage on the other leads, they are low voltage signals from the PCM monitoring circuits. the voltage generated by the O2 sensor itself, and/or stray voltage depending upon the scale setting of the VOM meter.

Confirm if the tune has been modified to FORCE open loop mode. If not, I suspect that PCM is still operating in open loop mode because one of the O2 sensors has been disconnected (the PCM can detect if the O2 sensor is disconnected or failed). As the O2 sensor is the most important sensor for closed loop operation, the PCM will NEVER go closed loop without both O2 sensors monitoring circuits returning good values.

Note: there are other sensors that are necessary for closed loop operation. When the PCM detects a failure in a required sensor, the PCM responds by going into "limp in mode". Part of "limp in mode" is to switch to "open loop" operation.

Open loop mode also happens at WOT.

I am not really sure that I understand exactly what you are trying to do. Nor do I understand what the current state of the problem is (IE what's been fixed and what were the results). Did you find and repair the cause for the blowing fuse (OP seams to indicate that you did)? Has the tune been altered to no longer FORCE open loop mode?

Tripoli · Feb 14, 2010

Your knowledge overwhelms me, and I appreciate your help.

I think I may have repaired the apparent short, as stated in my first post. I'm trying to fully confirm this before installing my supercharger and getting a new tune. I believe my current tune is set to force open loop. I will be going back to closed loop (hopefully) when I get the new tune for the supercharger in a week or so.

I've been driving the car for a couple weeks since (I think) fixing the short. In that time, the problem fuse which used to blow all the time has not blown, leading me to believe my problem might be solved. Since the car is my daily, I'm trying to make absolutely sure it's fixed before installing and tuning for the blower.

wmburns · Feb 14, 2010

Without knowing EXACTLY where/what was shorted, its not possible declare this problem absolutely solved. The reason is with a PCM FORCED OPEN LOOP tune, the O2 heaters will never be powered. So, if there is a fault in the heaters themselves, the fuse will still not blow.

IMO, based upon your description, the odds favor that the overload has been resolved.

If you need to know absolutely the problem is resolved, test the O2 heaters by grounding the returns and forcing the heaters on.

HO11(passenger front) return=RD/WH HO21 (driver's front)=YE/LB

The other option is to measure the resistance of the Heater elements. From what I can tell by looking at the pinpoint tests, if the O2 heater resistance is less than 5.0 Ohms, there is a problem with the O2 Sensor itself.

Good luck.

Tripoli · Feb 14, 2010

I could clearly use a few classes on how to use a multimeter.

The possible short I located was in the driver's side upstream O2 harness extension which came with my long tube headers. One of the wires (I'd have to get it in the air to see which one) had absolutely no shielding on it. Just bare metal wire. The only thing covering it was that mesh-looking stuff which covered the whole bundle of wires. I'm guessing that wire was shorting to the body or the transmission.

wmburns · Feb 15, 2010

Replace melted shielded cable

IMO the bare mesh covering is a grounding shield. It is supposed to be bare and its purpose is to reduce outside electrical interference to the wires inside. A far more likely scenario is that a wire INSIDE the braided shield has melted insulation and that wire grounded to the shield.

Therefore, if you “fixed” the problem by wrapping electrical tape around the entire cable, then the weak/broken insulation is still inside. It will lie dormant just waiting for the right circumstances of movement/vibration to re-occur (or it will wait until you are on a long trip ).

If the inside insulation has been damaged, I am afraid that the only real permanent fix is to replace the entire shielded line.

Tripoli · Feb 15, 2010

Actually, I did replace the line. I cut it between the connectors down to about 1/3", then spliced in a replacement wire. I then wrapped the ends in electrical tape and pushed the mesh back over the whole thing.

It's amazing to me that the wire had NO jacket on it at all. It wasn't melted off, it was just never there.

Upstream Oxygen Sensors - On with open loop tune?

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