Jeffrey H

On my 1996 LT4 engine I am scanning with a OBDlink SX with OBDwiz software. It presents 4 STFT readings, two look normal and two read 99% all the time. I have four oxygen sensor outputs that seem normal. Question is, does that engine have four STFT outputs? if so how could two be bad and the engine run fine? Are the STFT readings derived or are they sensor outputs?

Thanks

Cliff Harris

Your PCM is an OBD II system. It does not work the same way as an OBD I system does. An OBD I system continuously puts out a stream of data bytes. An OBD II system doesn't put out anything until it is asked. The software/scanner has to request RPM, MPH, coolant temperature, etc. and the PCM responds with only the data for the requested parameter.

What happens is that the scan tool is trying to cover every possible vehicle so it asks for ALL the parameters possible even if they are not applicable. OBD II supports 8 O2 sensors for example, so it will display nothing/garbage/zeros for the sensors that are not present.

Jeffrey H

I thought that since the scanner recognized the car and engine it was connected to than it would only request the sensors the car has and only display the data the car was capable of sending. As you say OBD II has 8 oxygen sensors but the scanner for my car only displays 4 so I would conclude that is all the car has so the scanner displays all the car has, and not all that OBD II has.

Cliff Harris

I was involved in writing an OBD II scanner program. What I did was display a blank if the ECM/PCM didn't respond to a request for data. I don't remember offhand if the ECM/PCM sends back an "invalid" answer in response to a request for data that it doesn't support.

I'm just trying to figure out why you see 99% for the STFT data. I looked at my code and it shows "NODATA" from the PCM if the parameter is not supported. The scan program ought to be able to react to that with the appropriate action (display a blank, dashes or whatever).

Jeffrey H

Yes! I would think that a NODATA reply would be the excepted reply, and not 99% for a no sensor reply, but that probebly depends on how the software was written for my particular scanner. Maybe I need to ask the forum on my scanner site.

kkperformance

Look at LTFT and STFT. the LTFT is a direct represtation of the STFT in a long term basis. You have only 2 banks, That is where you get B1 and B2. OBDII will represent live data just like OBD1. I have no exp with your scan tool but the 5 professional tools i own work as i described. You want LTFT and STFT +/- 5 to 10%. 0% IS IDEAL.

Jeffrey H

Thank you for your help.

Jeff