Originally Posted by PaulBlackFX4
Yeah I had a 87 perf tune from php when the light came on... I returned it to stock and reset the light and it came back on after a couple of days....Now the truck is completely stock and just had the plugs changed....it took a while longer but the light came back on yesterday. Im putting magnaflow 94136 cats on tomorrow. I will update later. Hopefully these new high flows will fix the problem as well as improve the sound of my exhaust.
Heres's what's goin on, in short. PCM Program: - The switch rate of the rear sensors are compared to the switch on the front. If the catalyst is efficient it will be removing HC and HO and the rear sensor will show a slow switch rate compared with the front. By dividing the number of downstream switches with those of the upstream gives a catalyst switch ratio, and a typical threshold is 75%, above which a DTC will be recorded.
It take a little while after reset because this test must be failed three times before the MIL (CEL) light is switched on.
Another thing, in case you don't know,-faulty HO2S sensors do not produce the DTCs P0420 and P0430. The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive (run).
So yea, 420/430 point right to the cats. There's other DTC's that point to the sensor, but you don't have them. There's also other DTC's that point to the harness, - you don't have those either.
So , a leak before the converter will force to much HO. A bad cat will force DTC.
You thought it may have been the plugs. That's nearly impossible because the Monitor Sequence program has to complete these tests before the Catalyst Monitor can run. It's called conditions or conditions met. - If I can remember -
1. HO2S monitor tests complete and no DTCs
2. You can't have any Fuel Trim DTCs
3. EGR Monitor Complete
4. No Misfire DTCs (which IS ignition of course)
I know I forgot one lol, shoot maybe two iduno...