Intermittant P0411 on my 2000 V6 Mustang

DaBard

Member
Jun 17, 2020
14
1
13
Hello, all, I have been battling an intermittent P0411 "Service Engine"/CEL light issue for some time.
Done the usual; replaced the smog pump. Reduced the frequency of CEL, but not completely.

So here's the story. Have a quick read, and comment.

History: this car was purchased in CT about 5 years ago, and has spent most time since in Upstate New York, where it snows a lot and they use lots of salt on the roads.
Would get P0411 with the CEL pretty regularly. So when I replaced the smog pump a couple years ago, I found that the wiring harness from the relay to the pump had corroded wiring. Obviously, the previous owner had been dealing with the issue. It looked like someone had put a tap on the wires to monitor whether the smog pump was being turned on, and the break in the insulation had let salt water in to rot the wires. So I fixed the wiring harness, put in the new smog pump, and things seemed hunky-dory...

...Until the CEL came on a few days later, code P0411. Not regularly; every so often. Tough to diagnose something that show up only every so often, and usually when you're not thinking about it.

So, I thought, why intermittent? Well, the P0411 DTC gets checked in the first minute or so after start, based whether fresh air from the smog pump into the exhaust stream drives the downstream O2 sensor to "lean". Well, if that signal doesn't go "lean" (low voltage), the the code gets set. So to pass the "emissions ok" test that the car does, there needs to be extra air (functioning smog pump and air delivery pipes), a "lean" signal from the O2 sensor (a good sensor), and a good signal path to the engine control computer.

Now, the smog pump can have an intermittent motor. The one I replaced ran ok when I tested it offline, but I left the new one in.

One thing that can cause the signal to fail to show "lean" is a ground fault, or some resistance in the ground path between the O2 sensor and the rest of the vehicle - like a grotty ground connection. Something which keeps the signal from being driven sufficiently low, every so often.

So I went around and looked at EVERY screw that connected a wire to the chassis. Every single one had corrosion on it. So I used lots of wire brush on a dremel plus contact cleaner to clean up the connection points, and replaced all the screws with clean new hardware. The crimp points of the wiring to the grounding connector itself are suspect, too.

So far, the CEL has NOT come back on, after several months. We will see once the car gets more exercise now that the snow is gone.

Now, the vehicle is 22 years old, and I suspect that the O2 sensors and cat converters are still original (180k miles on the odometer). The O2 sensors might be marginal. I plan to check all the O2 voltages, plus run some CataClean through the system this spring. Also renewing the grounding connectors, to make sure there's good connection to the wiring harnesses (plural) to the grounding connector. Shiny is good.

It's possible, particularly in older vehicles driven in salt-using states (like the NorthEast), that part of the P0411 problem might decay in the ground connections to the chassis. It's NOT on the diagnostics list, and is a relatively easy (and cheap) item to check out if someone is experiencing every-so-often P0411 DTC issues.

Comments appreciated.
 
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DaBard

Member
Jun 17, 2020
14
1
13
Update: the demon code is back this spring. Next steps is a can of Cataclean through the fuel system and some (ouch!) premium fuel, on the theory that the catalytic converter efficiency is marginal, but nor bad enough to trigger the P0420 code, and that the Cataclean perks up the O2 sensors.
Stay tuned...