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My P0420 and P0430 goes away with 93 gas

T

tbarney

New CEG'er
Joined
Jun 15, 2004
Messages
14
Location
Wisconsin
My 99 contour (about 105,000 miles) started getting P0420 a while back. I just keep clearing it with my palm scan tool and I am about to put mileliminators on to finally get rid of the problem. One thing I noticed is that if I run 93 octane gas for not even 1/2 a tank, the light goes off by itself. With gas as high as it is right now it isn't worth it for me to fill up with premium every time.

I'm guessing that the premium gas is running a bit cleaner and that is bringing the levels into an acceptable range. Does this show that I don't really have a problem other than the computer not accepting a wide enough range? I've read here that for a while Ford was reprogramming the vehicles to accept a wider range but of course I didn't have the problem until way after they stopped covering it. On the other hand, if this shows that I have clogged pre-cats then I might do something about that as well.

Thanks,
Tom
 
It most likely means your pre-cats are nearing their end of useful life. This does not mean they're clogging, just that the catalyst is no longer doing its job in treating the exhaust.

If your scan tool is capable of graphing the upstream vs downstream O2 sensors, look at that. A well-functioning precat will show little switching activity of the downstreams compared to the upstreams. As the catalyst degrades, the downstreams start to switch nearer to the rate of the upstreams and, when the ratio exceeds a preset limit, sets the fault code. A totally inert precat will show downstream sensor waveforms that look almost exactly like the upstream waveforms.

The 93 octane fuel should have no particular effect on the efficiency of the catalyst.

Tony's suggestion is the easiest and cheapest possible solution to the nagging of the MIL. Can be done for a couple of bucks (or less) if you can solder and turn a wrench.

Yes, there was a PCM reflash that widened the tolerance for setting these DTCs. However, I'm not sure if it applied to your particular vehicle. You'll have to check the particulars of the TSB.

Steve
 
Last edited:
projectSHO89 said:
If your scan tool is capable of graphing the upstream vs downstream O2 sensors, look at that. A well-functioning precat will show little switching activity of the downstreams compared to the upstreams. As the catalyst degrades, the downstreams start to switch nearer to the rate of the upstreams and, when the ratio exceeds a preset limit, sets the fault code.
Steve
Click to expand...

My scan tool tells me the voltage of the O2 Sensors. How do I interperate the voltages to look for as you call it 'little switching activity' ?

Thanks,
Tom
 
The tool has to have graphing capability. The actual voltage output of the sensor is retty hard to interpret otherwise.

Steve
 
tbarney said:
My scan tool tells me the voltage of the O2 Sensors. How do I interperate the voltages to look for as you call it 'little switching activity' ?

Thanks,
Tom
Click to expand...
Post what you have and we will look at it.
 
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