Code P0158

That indicates to me that you have a bad O2 sensor and need to replace it. Normally they fluctuate between .1 (lean mixture) and .9 volts (rich mixture). This is a heated sensor so when you initially start the engine it will not read until it warms up but you should not see more than 1 volt. Since you said that after it warms up you still only see fluctuation between .8 and .86 volts means the sensor is telling the ECM that it is stuck rich and no matter how lean the ECM makes it still will not register the .1 volts. If the ECM does not see a .1 volt after a certain number of cycles it will throw a code letting you know there is a fault.

Replace the o2 sensor and your should be just fine.

Thank you. Even after it warms up it will still go above 1 volt.

Yep! Time for a new one. In fact this is the 99.9% typical way these sensors fail.. to the rich side. Should cost you about $75.00 or thereabouts to buy the sensor.

Thank you. I thought my catalytic converter was on the way out.

Good afternoon.

Not so fast.

The downstream O2 sensor should never fluctuate. If it does, the catalytic converter is bad not the sensor.

It should stay steady and it is not stuck. It is doing its job.

This code is looking like a ghost code.

I would look into having your PCM updated with current information.

Roy

P0158
Descriptor
O2 Sensor 2/2 Circuit High
Probable Causes
Code Is Not Matching in Service Info File

Hopefully not! That is a very expensive fix! The fluctuations in the voltage that you see is just how the whole system works, the ECM constantly changes the Air/Fuel ratio to make it lean, then rich, the lean again because the catalytic converter can only do so much at a time, so the lean -rich-lean cycling by the ECM causes the chemical reaction inside the catalytic converter to switch between a reduction reaction (to get rid of nitrogen oxides or NOX for short) and an oxidation reaction (to get rid of the Carbon Monoxides or CO for short) so it sort of "breathes" like that back and forth the whole time you car is running!

So would a downstream o2 fluctuate if it was bad?

No it will not. As I stated, it indicates a failed catalytic converter if it does and it will set a code for a bad converter.

The voltage should never vary like the front. It only job is to monitor the catalytic converter and nothing else.

Roy

I still believe the o2 sensor is the problem. The code shows voltage high and he shows measurements of 1.27 Vdc, This should never happen with a good narrow band O2 sensor. This leads me to believe the sensor has failed. Otherwise I would expect to see a code for converter efficiency like a P0420. Considering the cost differential, (just looked up on AutoZone $35.00) wouldn't you think it was more economical to try this first compared to spending $400.00 to $500.00 on a catalytic converter? If I am wrong you can beat me with a wet newspaper!

Please do not confuse the customer with your responses. I never said anything about replacing the catalytic converter at all.

I listed the possibles for the code as you saw and there is no reference to anything which is possible that there is no issue at all. It may be that the PCM needs to be re-flashed.

I am sorry N1SGB2 for all the confusion.

Is your car running okay. Is there any performance issues?

Roy

Sorry, I guess I misread your post, I thought you were suggesting the catalytic converter was bad. You can take it from here.

I have always had it.

Roy

The car runs fine. The only issue is the p0138. The voltage stays within a range almost matching downstream o2 bank 2. It just switches quicker.

Just to be clear, you just stated code 138? The post list 158? Can you clarify?

Roy

The code 138 is for bank 1 sensor 2 but that has the same results I posted for the 158.

You need to stop comparing the upstream and downstream readings.

The upstream should be varying up and down. The downstream should be a steady voltage and again it does nothing but monitor the catalytic converter.

Roy

P0138
Descriptor
O2 Sensor 1/2 Circuit High
Probable Causes
Code Is Not Matching in Service Info File

Sorry I mean 158 downstream o2 bank 2.

Okay, same answer for that as well.

Roy

Thanks Roy. You think the catalytic converter is bad?

No, I do not. I do not think you have any issue at all. That code does not apply to anything.

That is why I think you need to have the PCM re-flashed with updated software which should remove that code.

Roy

Oh okay. Thank you.

You are welcome.

Always glad to help.

Roy

Thanks JonnyB1963 it was a faulty downstream o2 sensor.

Glad you have that resolved. Come back and see us again if you ever have the need!
Jon