STFT mis match and O2 sensor voltage issue

Rod (am I correct?), the pic below shows the signal lines for both a typical narrowband and also a wideband oxygen sensor.
In the pic above, the narrowband sensor wiring is on the left, and the wideband sensor wiring is on the right.

On a narrow band sensor, you have only two circuits. One is the power for the sensor heater, and the other is the signal circuit back to the ECU advising rich or lean. The heater circuit is used to bring the sensor element up to operating temperature. Until the sensor reaches operating temperature, it can not correctly signal the ECU about the engine fueling being rich or lean. Because of that, ECUs are programmed to ignore narrowband sensors at the cold start until enough time has elapsed for the sensor to come up to operating temperature.

The heater circuit in the narrow band pic above is powered by the solid black lines labeled heater terminals. The blue line is the signal line that the ECU watches to determine lean and rich operating conditions. When you start your engine, the ECU starts in open loop, ignoring the oxygen sensor because it is still too cold to respond accurately. After a few minutes of initial engine operation, and after the sensor comes up to operating temperature, the ECU switches from open loop operation to closed loop operation and depends on the oxygen sensor to signal rich or lean fueling conditions.

The right side of the pic above is a similar image but for a wideband sensor.

The red trace is the 0.5 volt sensor performance from Bank 1. The green trace is the 0.0 / 0.8 volt sensor performance from Bank 2. If the problem were a pinched wire or a bad connector, you should see no voltage. Your problem sensor's voltage swings between 0 and 0.8 volts. The good sensor's red trace hovers around 0.5 volts.


Because it represents a cheaper solution, I would first check the wiring for pinches. It is noteworthy that a pinch would take voltage to zero, not oscillate it between 0 volts and 0.8 volts, which you can see in the green traces in the pic above. The green trace, Bank 2, sensor should be providing a 0.5 volt trace like the Bank 1 sensor. The additional voltage and wild voltage swings seem to suggest an ECU problem — which is unusual.

Check the wiring and also check the connectors. If they all check out clean and/or unbroken, then you are looking at a more difficult problem. Ford has ceased production of the ECU some years ago, which complicates matters. If you live in a state like California with aggressive emission standards enforcement, then you will need a replacement OEM ECU to pass. If you live in a state with less aggressive emissions enforcement, you might be able to get by w/o the OEM ECU. The current regulatory emissions standards enforcement mindset has become increasingly rigid and less flexible/accommodating, so my bet would be you need an OEM ECU to pass.

One of the possibilities available would be a reman unit out of a corner auto parts store if the reman crowd is still rebuilding them. The other possibility is the use of an aftermarket ECU and replace it with a reman unit whenever you need a smog check. That, of course, presumes you can find a reman unit. Check your parts stores to see what they say about reman units. If you drill a dry hole with the parts store, let me know I might be able to help.

Although ECU failures are not that common, today, reman ECUs are also not that common. If an OEM ECU fails one or two years (or more) in the future, you are all but guaranteed there will be no reman units available, and you will be forced into a salvage yard search for a replacement ECU or scrap the car. That argues to get an aftermarket system, get the car running again and search out a replacement OEM ECU without the cloud of a timing deadline over your head to get the car running. When you find the replacement OEM ECU keep it safe and use it for emissions compliance testing, then put the aftermarket unit back in the car to drive it.

The good news, if there is any, is that the ECUs, for the most part, tended to be fairly robust and not prone to failure. Give the connectors and wiring harness a good inspection to look for physical damage. Your car's Bank 2 sensor oscillations between 0 volts and 0.8 volts, however, still troubles me ...

Rod,

When you buy the replacement ECU, it is important that it is for an 03/04 Cobra. The pinout on a Mach I ECU is visually identical to a Cobra ECU but with slightly different pin assignments than the Cobra ECU. I am not certain if the differences are actually hardware based or if the ECU has the ability to programmatically reassign the signals to the individual pins.

Second consideration; The ECU has what Ford calls a strategy which is Ford-speak for the equivalent of an operating system release for a Windows or Mac computer. The strategy the ECU was configured with will be on a tag that is pasted onto the ECU's wiring harness connector. The strategies can be 3, or 4 letter codes, sometimes with a numeric identifier following the letters. Some actual Ford examples are RYBE2C, YDH0, YDH1, AMZ1, AMZ2, and there are others. Each is tailored to a particular Ford engine car combination.

The original Cobra strategies were (if I remember the oldest ones correctly) YDH0 and YDH1. The final Cobra strategies were AMZ1 and AMZ2. If you get an ECU with a non-SVT Cobra strategy, it may not start the engine. Additionally, if the pinouts on the ECU connector are not software assignable, ECU strategy notwithstanding, it will be incompatible with your car's wiring harness and potentially damaging. The AMZ1 and AMZ2 strategies were the last strategies Ford released for the 03/04 Cobras. The differences between the AMZ1 and AMZ2 strategies have been lost in the corridors of my memory and were inconsequential in any event. Either the AMZ1 or AMZ2 strategy is the one you want to use.

Be sure to check the strategy name printed on the reman ECU wiring harness connector before you leave the store. Auto parts stores will typically not accept returns on electronics that have left the store — even if they were misassigned by the counterman. If you have difficulty finding an ECU with the correct SVT Cobra strategy, contact me by PM ('conversation' for this generation of website s/w). I have one brand new, never used AMZ1 strategy ECU from Ford that I will sell if you need it.

Ed
Thanks for all your help on this issue. I am attaching the label that is on the ECU. I am planning on ringing out the wiring this weekend. I have had the transmission and bell housing out 2 times the last several years due to clutch issues......RXT and a 26 spline so this issue is resolved...... so i want to make sure I didnt damage that bank 2 sensor wiring as it is on the same harness as the transmission sensors... I plan on getting to the ECU connector then as well.

eschaider said:

Rod,

When you buy the replacement ECU, it is important that it is for an 03/04 Cobra. The pinout on a Mach I ECU is visually identical to a Cobra ECU but with slightly different pin assignments than the Cobra ECU. I am not certain if the differences are actually hardware based or if the ECU has the ability to programmatically reassign the signals to the individual pins.

Second consideration; The ECU has what Ford calls a strategy which is Ford-speak for the equivalent of an operating system release for a Windows or Mac computer. The strategy the ECU was configured with will be on a tag that is pasted onto the ECU's wiring harness connector. The strategies can be 3, or 4 letter codes, sometimes with a numeric identifier following the letters. Some actual Ford examples are RYBE2C, YHD0, YDH1, AMZ1, AMZ2, and there are others. Each is tailored to a particular Ford engine car combination.

The original Cobra strategies were (if I remember the oldest ones correctly) YDH0 and YDH1. The final Cobra strategies were AMZ1 and AMZ2. If you get an ECU with a non-SVT Cobra strategy, it may not start the engine. Additionally, if the pinouts on the ECU connector are not software assignable, ECU strategy notwithstanding, it will be incompatible with your car's wiring harness and potentially damaging. The AMZ1 and AMZ2 strategies were the last strategies Ford released for the 03/04 Cobras. The differences between the AMZ1 and AMZ2 strategies have been lost in the corridors of my memory and were inconsequential in any event. Either the AMZ1 or AMZ2 strategy is the one you want to use.

Be sure to check the strategy name printed on the reman ECU wiring harness connector before you leave the store. Auto parts stores will typically not accept returns on electronics that have left the store — even if they were misassigned by the counterman. If you have difficulty finding an ECU with the correct SVT Cobra strategy, contact me by PM ('conversation' for this generation of website s/w). I have one brand new, never used AMZ1 strategy ECU from Ford that I will sell if you need it.

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