P0138 (O2 sensor) error - what next?

[4ndrewl]

Hi all,

We have a 2009 1.3 Toyota Yaris that has a seemingly unfixable problem.  The orange engine light has been coming on and our OBD tool reports this as a P0138 O2 Sensor High Voltage Bank 1 Sensor 2 problem. I can clear the message, but it always returns after a while - each time it appears it's always after a short, hard (for a Yaris) acceleration. The car drives smoothly and there seems to be no effect on performance either before or after the light comes on.

I've taken it to a Toyota main dealer who diagnosed the problem, replaced the O2 sensor, but the problem persisted. They wanted more money to diagnose it again 🙄
I've taken it to Halfords (UK-wide general garages) who said they didn't want to deal with it (it was outside of the usual simple work they usually do)
I've taken it to a local independent garage who also replaced the O2 sensor, and replaced the filters, carried out an engine flush. Still the error comes on and they said that's all they can do.

Hopefully someone will be able to help point me in the direction of what I could investigate next? I'm not really sure where to start - after the replacement of the O2 sensor, what's the next most likely things it might be?

Thanks any help,

Andrew

[TonyHSD]

Hi, 

did they replace the sensor after the catalytic converter? 
Because the fault keeps coming back it is likely to be from different things like bad catalytic converter or bad wire. 
These engines are well known for burning oil? How much oil is burning yours ? 
Are they any other symptoms like rotten egg smell, poor performance and acceleration, high fuel consumption, smell of raw petrol , all that can lead to bad cat, what is the total mileage of the car? 
And finally have you tried any fuel additives? 
If you haven’t you can add wynns catalytic converter and lambda sensor cleaner into your tank , add one bottle into max of 30ltr of petrol and take the car on a long motorway ride at leat for 40-50 min continuously driving at 60mph . This is vey good for any car once in a while as preventative maintenance. 
 

[Bper]

Hi Andrew,The likely cause of this problem is either damaged wiring or perhaps a small exhaust leak near the catalytic converter. As the error appears after hard acceleration, it would suggest a wiring fault or exhaust leak that becomes noticeable under load. High voltage codes like a P0138 can often happen due to a wiring short or grounding issue, and it’s possible that the engine movement during acceleration is moving a loose or weak connection, causing this intermittent fault. Even a tiny fray or loose connector can lead to high voltage readings, particularly under the added strain of acceleration.

A small exhaust leak near or just before the catalytic converter could allow extra oxygen into the exhaust flow, which may result in high-voltage readings from the downstream O2 sensor. These leaks are quiet often subtle but can be enough to confuse the sensor when the exhaust flow increases during any acceleration. Since the sensor has been replaced twice already, it may be a good idea to have a local garage carefully inspect the wiring around the Bank 1 Sensor 2, as well as check for any small leaks in the exhaust system.👍

[4ndrewl]

Thanks both,

Appreciate the replies - really helpful explanations. We're going to try a fuel additive (probably can't harm, but might just be temporary fix) and also look for someone who can analyse the wiring for intermittent shorts etc and look for leaks around the exhaust. Will report back for future searchers.

Thanks again!

[flash22]

what are your voltages and fuel trim readings, when a code is stored it will store freeze frame data too