Catalytic Converter Failure

Does the Electronic Control Module prevent fuel from being injected in a misfiring cylinder? If so, will this prevent damage to a catalytic converter if the wires are crossed or poorly connected or is it only designed to protect the catalytic converter until the vehicle can be serviced?

I have been having a problem with my 2000 Sienna Minivan. My catalytic converter needs to be replaced and I believe it is due to poor service from a service station (not a Toyota station). I brought my van in because of a check engine light. They ran a diagnostic test on it and told me it needed a tune up. They put it back together and it ran very rough. It bucked and would not go over 30-35 mph. It seemed to get worse as I drove but they may have been my perception. I called the station and they told me they “didn’t put it back together completely.” I drove it home about 5 miles and my wife drove it back the next day. She and my children noted noted a heavy odor of fuel. It was driven for about 10 miles total. They did the tune up and it ran fine except the check engine light came on again and a Toyota dealer confirmed the catalytic converter was burned out.

I have asked several different mechanics about this, including Toyota mechanics, and all have told me that the most probable scenario is that the ignition wires were crossed or not connected correctly at the diagnostic appointment and that it dumped fuel into the catalytic converter causing it to melt down. When I asked the service station about this, they claimed that the Electronic Control Module would prevent the fuel from being injected into a misfiring cylinder. I have reason to doubt their claim.

The engine computer does NOT control fuel injection differently from cylinder to cylinder — there’s not that level of sophistication, at least, not yet.
The Toyota dealer is wrong, also. It can’t be determined, with only a DTC (Diagnostic Trouble Code) that a component is faulty. I suppose that a lot of mechanics are never going to understand that.
A DTC only states that the expected voltage/frequency values of a sensor/actuator are out of allowable limits; or, that something has/has not happened when it should. The sensor/actuator CIRCUIT has to tested and checked to find where the fault lies. One does NOT jump to the conclusion that a sensor/actuator is faulty without doing the appropriate tests/checks.
Here is an abbreviated checklist for a code for the catalytic converter DTC P0420 (a commonly displayed DTC):

I don’t know about this particular car, but most engine computer setups these days do, in fact, trim the amount of fuel injected cylinder by cylinder. However, I don’t know of any that cut fuel to misfiring cylinders. Was your check engine light flashing while this was happening? It is supposed to flash to indicate misfire. You are supposed to stop driving if the light is flashing.

Sounds like the makings of a feature to add to the engine management program.

If a cylinder is misfiring badly enough to cause a blinking MIL then cut fuel off to that cylinder. However, don’t do this for more than one cylinder at a time.

It could save a converter or two with the people who ignore lights.

Thanks for the help. The light went on and my wife drove right into the station. She was literally around the corner when it happened. The total distance driven while the light was on was less than 9 miles before they touched it. I’m basing this on the fact that the shop is less than 4 miles from my house (when they told her it was okay to drive home) and that I drove it back and allowing for the (remote) possibility that the light was on for a mile before she noticed it. She was too afraid to drive it when the light went on and we got by with only one car until I got it back to the shop a day later. When I drove it back for the diagnostic, the van ran a little rough at first but smoothed out, similar to being cold. After the diagnostic tests, they put it back together so I could bring it back at my convenience. That’s when it ran so horribly (not drivable). I called them when I got home and they told me they didn’t put it back together completely. When my wife drove it back the next day, and this is when she and my kids noticed the fuel odor. It was driven a total of 8 miles in the extremely poor condition. This is when I believe the damage occurred. They claim that the Electric Control Module would have prevented the fuel from being dumped into the catalytic converter.

Rather than going by a code saying “bad converter” find a shop with a 5 gas analyzer and actually check your converters efficiency.

Have that shop show you the source of their info about fuel injector cutoff due to misfire. Don’t let them tell you fuel trim will totally shut off a injector,it doesn’t.

What is interesting is a PO420 (a generic code) will not set if a misfire or fuel trim code is present. Could this be to prevent a misdiagnosis of the cat when the problem is actually a misfire or trim condition. So if you get a PO420 you don’t have a misfire or trim issue (at least not detected)I am waiting for some better OBD2 info. so I can be corrected.

The problem is that the computer has to guess which cylinder(s) is/are misfiring. It does this by watching for slight changes in the speed of rotation of the crankshaft and noting what position it was at. Sometimes it gets it wrong. It can even be fooled by bumps in the road. I guess that on a V8, if it guesses wrong and cuts off fuel, you are only down two of eight cylinders, so it wouldn’t be so bad. On a four cylinder on the other hand…

Both shops are wrong. Well, the Toyota dealer may be right, if they did diagnostics beyond just reading the trouble codes. Hellokit gave an excellent explanation.

It is possible that the cat was damaged by driving 9 miles with a severe misfire, but as pointed out, the “Check Engine” light should have been blinking. If your wife didn’t report the blinking to the shop when asking for advice, the question of liability gets complicated.

How many miles are on the van? The cat may have been near the end of its useful life anyway, and the misfire may have just been the last straw.

“On a four cylinder on the other hand…”

The computer should be able to tell it has chosen the wrong injector to disable.

Then it could try each of the others in turn until it finds the one that does not make the loss of power worse.

It could do all this in a few seconds. And again, all this would happen only if the misfire is persistent enough to trip a blinking MIL.

Thanks for the reply. The check engine light went on (not blinking) and my wife took it straight to the shop. The van has 70,000 miles. They told her it was safe to drive and the light stayed on until the tune up was done. I think I may have confused people with the time line, so I will try to clear it up:

Day 1: My wife is driving by the shop and the check engine light goes on. She drives right intot he station. They tell her the car is safe to drive.
Day 2: Car stays in garage, no one drives it.
Day 3: The light is still on. I bring it in for the diagnostic. After 2-2 1/2 hours in the shop, they tell me it needs a tune up. I arrange to bring it back the next day. The light is still on. I take it home and it bucks and jerks the whole way home. I call and the mechanic tells me he didn’t put it together completely but it’s not a problem because we’re getting it fixed the next day.
Day 4: It stays in the garage until my wife takes it in to the shop when I can pick her up after work. She and my kids smell fuel all the way back to the shop.
Day 5: They do the tune up and the light is now turned off for the first time. We pick it up and it runs fine.

One week later, the light came on again and they turned it off again. It came on again right away and I took it to a Toyota dealer who confirmed it was tha catalytic converter.

They claim the damage was not related to the misfiring between the time they diagnosed it and the time they fixed it. I claim they mixed up the wires or damaged the wires and caused fuel to be dumped into the cat.

Again, thanks for all of the help.

Perhaps you should supply the algorithm for this to the automakers. It’s not that simple.

I agree with your claim. They [the garage that mis-wired your car] are the cause of most or all of the damage and should pay for the converter replacement. However, good luck collecting on that.

One more time, if the Toyota dealer did nothing beside read the code, they have no basic for declaring the catalytic converter toast. If this is the case, you still need a decent shop who does, and understands, diagnosis.
The catalytic converter is not a goner until it has been diagnosed by a doctor (qualified mechanic). What DTC (Diagnostic Trouble Code) is being displayed?