View Full Version : Cat Converter Temp sensed in 4 wire O2 sensors!!

December 2nd, 2017, 16:11
While using my Snap-On MT-2500 tool on my 2001 Saturn today, I made a cool discovery.

The scanner reports the actual Cat Converter operating temp!!!:cool:

Only one way that is possible folks!!! In 1987, Renix the O2 sensors had only 3 wires!!! IN 1991 OBD-1 switched to 4 wires (I think) and in 96 all OBD-II rigs I have ever seen have 4 wires. The 4 th wire is a second ground

I have always been puzzled about that second ground.

Now I know!!!

The second ground is dedicated to the electric heater in the O2 sensors.

That way the ECU/PCM can monitor the current going through heater (via the ground wire, grounding through the ECU circuit I think, noise free) and the current is proportional to the Cat Temperature!!!!

It turns the heater wire into a dual purpose gadget, called an anemometer folks!!!

Today I was sitting there watching the Cat Converter temp live on this Snap-on tool!!!

It was running at 585 F at idle and shoots up 650 F when I gun it in park for about 10 seconds. Great way to see if a Cat is dead or not, no doubt that is part of how OCD-II knows to throw a bad Cat Converter code folks!!!

These old MT-2500 scanners that cover 1984-2001 are dirt cheap on ebay now as the shops are dumping them to upgrade. Great way to help diagnose failed emissions
especially in Ca. You can watch the CPS, TPS, MAP IAT, IAC, O2 sensors, data all live with these, and so much more. Even the missfires per cyl data and spark voltage data.... and so on, find stuff long before a code is thrown.

December 2nd, 2017, 20:22
The heater current can indicate the temp of the O2 sensor, which can tell the ECM if it's hot enough to switch to closed loop mode. If it's a wide-band sensor, it can use the temp to do some calibration of the sensor. That sensor is also probably upstream of the cat. :}