2005 Nissan Altima Catalytic Converter

The oxygen sensor in the exhaust system switches between rich and lean conditions very quickly, up to a few times per second. Since 1996, there has been a second O2 sensor after the catalytic converter. If the converter is working proprly, that second sensor will switch between rich and lean very slowly, perhaps many seconds between cycles. When the converter stops changing the exhaust gases into something harmless, the stuff going out is the same as the stuff going in. The second O2 sensor switches at the same faster rate as the front one. That's how the Engine Computer can tell when the converter isn't dong it's job. That alone should not damage the engine.

The converter can also become plugged from leaded fuel in the past, and from the catalyst melting today. Too much unburned fuel in the exhaust will overheat the converter and melt the porous material causing it to become plugged. The typical symptoms are very low engine power, engine overheating, an unusual hiss from the tail pipe, and possibly some popping out of the intake system. The engine will also have a tendency to idle unusually smoothly.

Before you have the converter replaced, the emissions system should be inspected and tested first. Many more problems are caused by the oxygen sensors than the converters.

Caradiodoc

Hi Joeyjimz. What's your thinking? Are you saying your head gasket now was caused by a plugged converter 8 months ago, or how are they related?

A glowing catalytic converter is a different problem with a different cause than what ISB68 has. Too much unburned fuel is the reason for the converter overheating. That will melt the catalyst and cause it to become plugged. If you have a '96 or newer car, the Check Engine light should be flashing. The excessive fuel can be caused by a single cylinder misfire, spark related, excessive fuel pressure, or an injector stuck open.

A plugged converter will not cause a head gasket to leak, at least not directly. If engine power is reduced, as you press harder and harder on the gas pedal, the engine works much harder to push the exhaust out. It starts to heat up like an air compressor. The higher engine temperature could cause parts to expand more than normal, (aluminum heads in particular), leading to a warped head or leaking gasket.

Since the head gasket was replaced, do you feel a misfire to go along with the red converter? The glowing should be addressed before the new converter melts inside. One thing that is often overlooked, especially when there is no noticeable misfire is an exhaust leak ahead of the front oxygen sensor. In between the pulses of exhaust gas, the momentum creates a pulse of small vacuum. Air will be drawn in through the leak, and the oxygen sensor will detect unburned oxygen. The Engine Computer will command the injectors to stay open longer to add fuel to go with the oxygen. No matter how much fuel is added, there will always be that unburned oxygen there so the computer keeps increasing the injector on-time as much as possible. You might smell raw fuel out of the tail pipe, but O2 sensors don't detect unburned fuel, just oxygen.

Caradiodoc

I'm not going to pretend to be a Nissan expert, but I can give you some generic suggestions. The only way a catalytic converter can get red hot is by burning too much unburned fuel. That will lead to a melted and plugged converter eventually. The question is why is there too much fuel?

The most common cause of too much fuel is because the Engine Computer thinks the mixture is too lean so it turns the injectors on for longer pulses. A misfiring cylinder due to loss of spark will allow unburned air and fuel to enter the exhaust stream. The oxygen sensor will detect the unburned oxygen and make the computer think there isn't enough fuel to go with it. O2 sensors don't detect unburned fuel. No matter how much extra fuel is commanded by the computer, there will still be that unburned oxygen. The unburned fuel from the misfiring cylinder, and the extra fuel from all the other cylinders burns in the converter.

A similar problem occurs if an injector isn't firing properly or is disconnected. There will be a bunch of unburned air entering the exhaust that is detected by the O2 sensor. The computer will increase the fuel from all of the other injectors. Now you have a bunch of fuel and air again that burns in the converter.

A much less obvious and hard to find problem is a leak in the exhaust system ahead of the O2 sensor. In between the pulses of exhaust flow, the momentum creates small pulses of vacuum. That vacuum draws in outside air that is detected by the O2 sensor. The computer increases fuel metering in an attempt to overcome the lean condition, but no matter how much extra fuel is delivered, the unburned air is still there.

A different but related problem can occur on cars with distributors. If the ignition timing is excessively retarded, the mixture will be ignited and start burning too late. It will still be burning when it is pushed out past the exhaust valve. This will be most visible on engines that have thin header-type exhaust manifolds. They will glow orange, but the burning is usually done before the mixture gets to the converter. Engine power will be low too.

On engines with a vacuum hose going to the fuel pressure regulator, a leaking or improperly connected hose will cause fuel pressure to go too high. That will force more fuel through the injectors than the computer is expecting. If that hose is allowing air in, you'll have extra fuel and extra air. Normally all the fuel burns in the engine, but idle speed will be too high.

As a rule, a problem with a single cylinder is usually the cause of too much fuel and air entering the converter. The clue is that idle speed is close to normal.

Caradiodoc