Questions and Answers
GMC Sonoma DTC P0420
Q. I have a 1997 GMC Sonoma, 4x4, 4.3 liter V-6 VIN code"X" motor (190hp) with 5-speed manual transmission with 77,000 miles. The truck is single exhaust with all factory components. The Service Engine Soon light came on and is showing a DTC of P0420; "Catalytic System Efficiency Below Threshold (Bank 1)"
The truck is not running rough nor has the fuel mileage been effected. Three months ago new plugs, wires, cap and rotor were installed along with a radiator flush. A new coil was installed a year ago. I have tested the catalytic converter by checking the inlet temperature vs the outlet temperature and I get more than a 100° F difference telling me that it is okay. Plus, it passes the rattle test; the catalyst is still intact and not rattling around inside.
I then moved on to testing the heated O2 sensors. There are two before the catalytic converter and one after. All are 4 wire units and are identical. All were tested on the bench with a Fluke 88 DMM (digital multimeter). The first test was to heat the tip with a propane torch for 20 seconds to see if it gets to 0.6 volts and drops to zero or very near zero when the torch is removed, within 2 seconds.
The second test was to heat the sensor with the propane torch for 2 minutes and check for voltage drops. No voltage drops and it did not go over the designated 0.9 volts during the 2 minute test. All three sensors were tested and had quick reaction time on the DMM. I am unsure what to look at next and would like to know what your experience tells you to check next.
Thank you for your time.
MattA. This code is pointing at the catalytic converter. Remember this is OBD-II and they are usually right on the money with calling a failed catalytic converter. Unless there is an updated EEPROM re-flash to cure this code, sounds like you need a new catalytic converter.
