Someone has suggested to me that the cause of my code "41 c no hego switching detected" (92 F-150 5.0 so it's eec-IV) could be problems with the catalytic converters being blocked somehow (I've heard a faulty O2 sensor can cause that in the first place, code 41s detecting lean conditions, pumps in more gas, catalytic converter collapses inside coping with the excess fuel)

Certainly might explain the lack of power and behaviour that seems a little like missing...

The case put to me sounded a lot like mine and this person had changed out just about everything....so far I've avoided it by testing everything I can (still lots to go) - I'm glad I bought a vacuum pump- and was wondering how I can check the cats in situ- took it to a couple of exhaust type places and neither seemed too enthusiastic about it, and doubted the cats were bad. Whilst I'm glad they think I don't need to replace them, I want to find a concrete cause and therefor solution.

What was suggested to me was to measure the temp of the exhaust system in various spots, before and after both cats. The idea is that the pipe will be cooler upstream of the cats. Makes sense although it won't detect blockage but will check if cats are still causing unburned fuel to combust (although unless a platinum thief has been inside I'd have thought it'd still work albeit less effectively) So I bought an infrared non-contact thermometer, I figure the best approach is to do it when the system is cold and have it on ramps, lie under it on a dolly and read the temps straight after it's started then maybe down the track too (I figure if it's warmed up and sitting after a while then the temp along the pipe will even out.

Any better ideas?

 

Use your IR thermometer to monitor inlet and outlet pipe temps from a cold start. You should be able to compare the two sides and see when the cats light off.

Faulty O2 sensors can lead to cat failure. If they're reading "lean" or there is a misfire, too much fuel gets dumped and the cat eventually gets overloaded. As long as the honeycomb hasn't collapsed and caused a blockage, the failed cat will have no effect on driveability. If you are experiencing symptoms that indicate a blockage, you can remove the upstream sensor and see if the engine's ability to breathe is improved.

Steve

 

Hmm there's a thought, that'd be a cheap easy way to do it, when you say "remove" do you mean remove the sensor, or just disconnect it? If it's removed or disconnected, of course I'll get codes, but you're saying it ought to run better? That's a great idea, the best suggestion I've had so far....the KISS principle writ large...

 

how many o2 sensors does your truck have? mine only has one and its before both cats. so i just cut the pipe right after the front cat and took an air chissle and gutted the cat for louder exhaust. then i welded the pipe back together. i dont have a cel so if you have the same setup then you may be able to try this as a temporary fix. or just cut the back of the cat and shine a flash light through to see if you can see through it. if you can then dont just weld the pipe back yet, try starting it and if it runs better then its the rear cat. just a thought.

 

I have just one sensor, upstream of the cat...at least I've only found one sensor! Be funny if there was a second one, after all I've been through....

Seriously I believe there's only one. I don't have welding equipment, I'd thought of the idea of trying to bypass both converters, I doubt the neighbours would appreciate it since the simple way would bypass the muffler too. I guess some sort of temp pipe that ran to the muffler would be the way but my 'workshop' is kind of non-existent, I do have a reasonable collection of tools but nowhere to work and no real metalwork stuff.

Now I know how to check the cat, I can do that if I can get the exhaust apart (my driveway is sloping, another challenge since I don't want to work in the street) - I found a great article on cats : http://www.aa1car.com/library/converter.htm

I kind of want the quick fix, but want to find the source of the problem, which means checking the tb/cleaning it, and the iac as well. I suppose pulling all the plugs and checking them too would be a good idea, might indicate a valve problem or similar, that would account for a clogged cat, assuming one is.

One thought springs to mind - assuming the cats are the same age, the forward cat did seem to be working (and, perhaps working too hard looking at that article) how could the rear one have clogged/failed before the forward one?

 

well the front cat could be melting when it gets hot then all the crap goes into the second and plugs it up.

 

Remove the sensor.....If the CAT is clogged the hole from the vacant O2 sensor will free up the exhaust.......

 

well I just swapped out my entire exhaust for some OBX long tubes, Maggie high-flow and maggie cat-back. When I cut my cats and pre-cats apart you could see the "honeycomb" was half melted and manlged. It could be a possibility for yours as well. I'm sure for my truck it was a combo of alot of raw fuel dumping in my hot exhaust (because I was trying to figure out a COP misfire) and pulling heavy loads up mountain Highways floored for extended periods.

I hope you figure it out, keep us posted on your results.

 

Makes sense re the clogged exhaust, a lot simpler just unscrewing the hego, I'm just wondering what it'll do when it detects all that oxygen....if I left it unscrewed but connected. If it's disconnected I suppose it's the same as what I'm getting now - no switching detected? Would I be better off with one of those gadgets that simulates a good mixture?

That cat article suggests testing the overall engine vacuum should read I think 15-18in Hg, that I can do easily enough (is the brake booster line the best to use? I assume it's a straight line, no delays or modifiers in there?)

 

Engine vacuum levels appear ok, on to the next things - cleaning tb and checking plugs