Prius Gen 2 P0136 P0138 Dtcs

[ColinBarber]

Recently had to look at a Prius which had error codes P0136 and P0138. These codes relate to the rear oxygen sensor, P0138 indicates high voltage. Normally you would expect the sensor to be fault and needs to be replaced however in this case it was the upstream sensor at fault.

Ideally try and check to see if the downstream sensor is working correctly, at least giving some high and low readings whilst driving. An alternative is to check the freeze frame data, if the downstream sensor voltage was between 0.5 and 0.9v then it is probably ok. If it is stuck at maximum voltage then it probably is the downstream sensor at fault.

It seems the upstream air/fuel ratio sensor gets lazy or out of calibration and runs the engine rich. When the ECU performs an active sensor check (designed to purposely make the downstream voltage go low and high to confirm correct operation) it thinks it is running the engine lean but isn't and therefore the downstream sensor voltage remains high and a P0138 is produced.

Replacing the upstream sensor fixed the problem. The sensor is slightly difficult to replace, you need an offset sensor socket and you will probably have you grind the outside of the socket down, unless you use the genuine Toyota tool. This is because the sensor has a heat shield around it which the socket has to fit inside. The Toyota service manual states you need to remove the hybrid inverter. Not really sure why they state this, if you remove the wipers, motor and top cowling (as you would if you are replacing the plugs) then you can easily get to the sensor plug without removing the inverter.

I was quoted around £220 for a new air/fuel ratio sensor from a Toyota dealer (89467-47010). Instead I used the Denso OEM product, part code DOX-0262 which can be picked up for under £100. Normally the Denso aftermarket sensors are designed to fit a number of vehicles and therefore the leads can sometimes be a little long, in this case this particular sensor is only used for the Prius and therefore is an exact identical replacement. The only different I could tell was an area on the sensor which seemed to have been rubbed down, I think to remove the Toyota stamp and code.

Looking on some of the US forums it seems it isn't uncommon for this error, and it seems most dealers are replacing the downstream sensor first and then having to replace the upstream at the second attempt to fix the problem.

Hopefully this info will be useful to someone. The Gen 2 Prius are getting old enough to start seeing these types of issues.

[timberwolf]

Thanks for the info.

A couple of questions come to mind.

How would I tell my dealer to do their job properly when diagnosing that fault? i.e.

The repair manual does say that those DTC could be due to either sensor although it does mention the Oxygen sensor a lot more often than the A/F sensor. It provides a method using the tester to determine the sensor that is the cause of the problem - would that mean the dealers not following the diagnostic procedure?

I've always wondered where a customer stands, If a dealer does replace a part and it doesn't fix the fault, can the customer demand their money back from the first repair?

[ColinBarber]

I did read that somebody was told by their dealer that Toyota had updated their procedure. Not sure if this is true but I did find the attached for a different type of engine which does indicate the upstream sensor as the first thing to try for a P0138 in certain circumstances (page 4).

This is all within the US, I'd like to think the UK dealers get the same info as well.

The standard repair manual states to replace the downstream sensor first which is what dealers were/are doing. The problem as I see it is that if it isn't a very clear cut fault the test procedure can be inconclusive and therefore you have to just eliminate things by swapping out. Hopefully once a dealer has got it wrong once or twice they will learn, this is normally an advantage to going to a franchise dealer - they tend to experience many more faults, and more frequently, than independents working on several makes and models.

As to who pays if they don't get it right first time, that is a difficult one to answer. You shouldn't have to pay for the item if you don't need it but it cannot be put back into stock and sold as new and additional labour would be required to swap back to the old part, if they even kept it. Paying for the original labour, I guess it could be argued that they are following the steps to diagnose the issue.

Again looking at experiences in the US, dealers are doing the second piece of work with FOC labour or parts at cost. I guess it is really down to a discussion between the dealership and owner, they need to do what it takes to keep you as a customer.

P2195_0138T-SB-0066-10.pdf

[colmenedo]

Hello! Regards from Spain Pardon for my alone Englishman thanks for this post the author since in my prius it was a ruin P0136 replaced 1 catalyst and 4 sensors of oxygen Today after teaching this post to the technician of toyota suspicion of an evil has put to read values of the sensor a/f warming of this sensor in cold that tomorrow he was verifying. You mantendre informed Thank you and a greeting