Toyota Siena Malfunction Indicator Lamp keeps coming back

I posted this question under the term ?engine light? the week before and got a few suggestions, but the problem has not gone away.

The Malfunction Indicator Lamp of my Toyota Siena CE 1998 keeps coming back. After the garage replaced oxygen sensors and the catalytic converter, it came back. They replaced again the oxygen sensors, adjusted the distance of the catalytic converter (or the catalytic converter doesn’t come up to full operating temperature, as I was told), it came back. I took it to a Toyota dealer who diagnosed that the problem came from the catalytic converter. The garage said the new catalytic converter, a genuine Toyota part, is defective and replaced it again. Three weeks later the light comes back.

I took the car to Advance Auto for a scan, as suggested by a gentleman from the NPR Car Talk community. The result is “catalytic efficiency below threshold, Bank 1”. I was told that the front oxygen sensor was either defect or installed inproperly. After the code was reset (P0420), the lamp went off briefly, but lights up again the day after.

The seat-belt light on the display panel has been on for years despite the fact that we all wear the seat belt the moment we get into the car. The garage does not think the display panel or wiring affects the engine light.

The garage has been willing to follow-up without charging me more (after I spent about $2000 on replacing the OS and pipes). If I go elsewhere now I’ll have to pay again, but I feel bad keep going back to the same garage. Any advice?


Has your mechanic looked at the signal the O2 sensors are putting out? Maybe print this article to show him:

I agree with Circuitsmith about having the shop look at the O2 sensor signals using a scope to see what is going on.

You are basically commited to have the shop finish the job. You have gone way past the normal cost to repair this problem. I would be pretty upset paying that much money and still have no results.

The vehicle’s engine monitoring system keeps track of things that have to do with emissions mainly. The seatbelt warning light is not part of that system. You need to check the wiring to the catch for a problem.


I gave the garage your suggestions. They said they’ve worked on all of these problems already. While promising to go over the list again, they don;t seem to be optimistic in finding new solutions. What happens to the car if I keep driving it with the engine light on? Someone says nothing would happen, the car won’t be damaged. If that is the case, why does the manufacturer’s manuel tell owners to bring the car to the dealer when the light is on?