2003 Toyota Sienna Check Engine Light

In June of 2009 the check engine light came on with this van with 113,658 miles. We took it to a Toyota dealership and the scan performed indicated code P0420 - needs convertor and o2 sensor. Cost estimate of $600-$700. We declined the service and the code was reset.

January 2010, 13,000 miles later, the check engine light comes on again for the first time since June 2009. Took it to Autozone and their scan showed P0136 - which has to do with the o2 sensors.

We live in New York State and the van has to be inspected this month. Any suggestions on what approach we should take or has anyone experienced this problem?

Also, when the dealership resets the code is it the same as when we disconnect the battery to clear the check engine light?

Thanks for your help.

The shop manual for the 2002 Sienna says for 0420:

Catalyst System Efficiency Below Threshold.

Things to check:

*Three way catalytic converter
*Open or short in heated oxygen sensors (bank 1, sensor 2) circuit
*Heated oxygen sensors (bank 1, sensor 2)
*Open or short in A/F sensors (bank 1,2, sensor 1) circuit
*A/F sensors (bank 1, 1 sensor 1)

For P0136 it says:

Heated Oxygen Sensor Circuit malfunction (Bank 1 Sensor 2)

*Heated Oxygen sensor

The troubleshooting page for this says if no other codes present, chack for open and short in harness and connecter between ECM (computer) and heated oxygen sensor, if bad replace harness. If okay, check output voltage from sensor, after making sure the connections are all solid.

If it were me, I’d go look over harness, and replace the sensor.

People here tend to be all over the place. A while back, I asked when those sensors tend to start failing, and was told by one or more people, they never fail. Alas, I already had had one fail at about your 115,000 miles, and since I use my car in Mexico far from repairs, wanted to know when to replace the other two. I thought it over, and had the other two replaced, which I hope reduces chances of failure for quite a while.

So, my guess is if no visible harness damage and connections are good, replace the sensor, period. Be sure to get the right one if you need to be smog checked. This is not the time nor place to guess a cheap universal sensor will work. On mine, the correct one was somewhat expensive, and googling showed the correct one was indeed expensive no matter who I bought it from.

P0136 is for “low voltage output from the rear oxygen sensor”. It could be a faulty oxygen sensor, or, it could poor wiring conduction between the oxygen sensor and the engine computer. Diagnostics to check out the voltage might be equal to the price of a replacement oxygen sensor. So, it’s your choice.