Hey. I have a 98 1500 5.9 gas. 4x4. This is the second time I get this pair of codes set. P0420 catalyst efficiency below threshold and p0720 TCC circuit malfunction. Truck was running fine today but had the light on. New NKT o2 sensor a few months ago. Didn’t notice any drive ability issues. Just a note also had the abs and brake light on at the time.Any ideas on the codes, especially since they set together twice now.

 

Is there still a cat on it? Does it still have its guts??

2 or 4 wheel ABS? Your speedometer working properly? What about cruise control?

 

The scanner showed 88C which is 190F.

Can you clarify if you had p0720 which is a speed sensor code or a different code related to the TCC ? And they always pop up together ?

 

P0720 would be Low speed senor above 15 mph, If you had a TCC code that could be P740 TCC no rpm drop or maybe P0743 TCC solenoid code. I would try replacing the speed sensor on the drive side on the tail housing of the transmission. Be care when the sensor is removed it will leak fluid.

 

I do have a Cat and I believe it has been gutted, 2 wheel ABS. Speedometer and cruise are working fine.

 

My bad on the temperature, I edited that out of my post after i realized my mistake. I was going off the scanner I have which says 0720 is TCC malfunction, but more than likely it is a code for the speed sensor. This is the second time my check engine light has come on and both times I get those two codes together. Runs fine and don't notice anything out of the ordinary, except the money light is on.

 

The gutted cat will give you the P0420. Do a search for 'non-fouler trick' in the FAQ/DIY section. Couple threads in there about it.

The P0720 is talking about the output speed sensor on the trans.