setting code P0138 ***finally fixed***

[72chevman]

This code appears to be related to my rear O2 according to my service manual. I find this hard to believe as I have never seen a rear 02 go bad in any GM product I've owned. Has anyone else seen this code & how did you repair?

I've owned the car for 180 miles now. It is a 96 with a 3.1 and 87K.

I've replaced plugs, air filter, fuel filter.
AcDelco wires are on order with rockauto.com
Fuel pressure is 42 with key on engine off and 38-39 with engine running
This first full tank has a bottle of Techron FI cleaner in it.

[Cliff8928]

DTC P0138 HO2S Circuit High Voltage Sensor 2

Circuit Description

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors. The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor. If the HO2S 2 signal voltage remains excessively high for an extended period of time, DTC P0138 will be set.

Conditions for Setting the DTC

No active misfire, fuel injector circuit, TP sensor, EVAP, IAT sensor, MAP sensor, Fuel trim, EGR, ECT sensor, MAF sensor or CKP sensor DTC(s) present.
Engine coolant temperature above 75°C (167°F)
Throttle angle between 5% and 40%.
HO2S 2 signal voltage remains above 999mV during normal closed loop operation.
OR
HO2S 2 signal voltage remains above 800 mV during deceleration fuel mode operation.
Either condition for up to 50 seconds.

Check for the following conditions:

Fuel pressure. An excessively rich fuel mixture can cause a DTC P0138 to be set. Refer to Fuel System Diagnosis.
Rich injector(s). Perform Injector Balance Test.
Leaking injector. Refer to Fuel System Diagnosis.
Evaporative emissions (EVAP) canister purge. Check for fuel saturation. If full of fuel, check canister control and hoses. Refer to Evaporative Emission (EVAP) Control System.
MAF sensor. Disconnect the MAF sensor and see if rich condition is corrected. If so, replace MAF sensor.
Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel.
TP sensor. An intermittent TP sensor output will cause the system to go rich, due to a false indication of the engine accelerating.
Shorted Heated Oxygen Sensor (HO2S). If the HO2S is internally shorted the HO2S voltage displayed on the scan tool will be over 1 volt. Try disconnecting the affected HO2S and jumper the HO2S low circuit to ground with the key ON, engine OFF. If the displayed HO2S voltage changes from over 1000mV to around 450mV, replace the HO2S. Silicon contamination of the HO2S can also cause a high HO2S voltage to be indicated. This condition is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is evident, replace the affected HO2S.
Open HO2S Signal or Low Circuit or Faulty HO2S. A poor connection or open in the HO2S signal or low circuit can cause the DTC to set during deceleration fuel mode. An HO2S which is faulty and not allowing a full voltage swing between the rich and lean thresholds can also cause this condition. Operate the vehicle while monitoring the HO2S voltage with a scan tool. If the HO2S voltage is limited within a range between 300mV to 600mV, check the HO2S signal and low circuit wiring and associated terminal connections . If the wiring and connections are OK, replace the HO2S.

[3X00-Modified]

Rear o2 is actually a very common failure or code for that matter, and as Cliff pointed out with his post, it can also be something else that went wrong and the sensor is simply doing what it should and notifying you of the problem elsewhere...

OR if you can check it, check and make sure the cat is ok and not plugged up or anything. That will also trigger the same code.

[72chevman]

This car sometimes has a bog or stumble at lower rpm when you drive it nicely. That is normally an indication of a bad O2. I wonder if the front O2 is bad and the code is setting cause the front & rear can't see "eye to eye". If you give it good gas, the car does not stumble, it just goes, so I don't see the cat as being clogged. Would the car not have good power and the cat get really hot if it was clogged?

How do they check for a clogged cat these days? We used to drill a hole before & after the cat and do a PSI check on the exhaust pipe. A small drop was normal, a huge drop meant the cat was clogged.

A bad front O2 could explain my poor MPG too. The 5 plugs I have pulled looked pretty good, a little black, but not that bad.

[72chevman]

Well, I changed both 02 sensors. There were pretty cheap thru rockauto. I went with a universal rear and spliced it in. The front I went with Denso which was like $32. I still got the code two days later instead of that day, but the car has not stumbled and the MPG has jumped, so I know the front was probably somewhat bad and giving poor readings. I'm guessing next is cat. Anyone know if I can rent a backpressure tester anywhere? I've never tried that. I guess they screw into the 02 sensor bung. The factory manual says 1.25 PSI at idle and 3 PSI at 2000 is normal for the front and rear if all is well.

[3X00-Modified]

Never tested it that way... I've always just removed them as a whole and seen if they rattle around... Just unbolt it from the manifold and the 3 bolt flange and see if she makes noise... if she does its all broke up.

[72chevman]

I'm at my wits end now. I changed the converter and still get the code. I guess I will take it to my buddies shop with a high dollar diagnostic machine to figure this out. I pride myself with doing my own maintenance, but I guess not this time.

So far this is what I have done.

Fuel pressure within specs at idle and as RPMs go up.
New plugs & wires, fuel & air filter
New O2 sensors
New Catalytic converter
No fuel in pressure regulator
Cleaned all TB ports and all sensors with TB cleaner

[72chevman]

I took it to a shop. They found corroded grounds that were giving bad data to the pcm from the O2 sensors. Once that was fixed, the mass air was also bad as well as a crack in the PCV line so it was leaking vacuum. Out the door was $360.

Engine diagnostic fee $79 but that covered finding & repairing the grounds.
Re-manufactured mass air was $199 + installation
New PCV hard pipe was $24 + install.

The car runs much better now that the PCM is not getting data that it is lean most of the time. I'm sure that is why my MPG was poor. The pcm kept adding fuel to cure the lean signals, but the lean signals were due to bad grounds and bad mass air.

[Money pit Beretta]

Good to hear that you are up and running again! It may be a good idea to put some SeaFoam down the brake booster hose. That would clean out all that build up from running rich. If you are not sure how it's done feel free to ask. I've done it, as have many others here. Here is a link to look at
http://www.beretta.net/forum/viewtopic.php?f=9&t=1031

[72chevman]

I'm an old hat at mechanics. Been working on anything that goes fast since I got my first go-cart at age ten in 1979. I actually prefer BG 44K for cleaning the FI system. Seafoam is good, but professional shops around here use BG 44K with their FI system cleaning service. I know of a local dealership that treats any used car that has rough idle issues with this. Add a can to 1/2 tank of gas and use the car as a runner for a few days. Normally cleans it right up. Yes, it is $22, but worth it.

[Money pit Beretta]

Never heard of it or seen it. If I do see it I'll be sure to get some.

[72chevman]

Only sold at repair shops normally. They do not sell it at Walmart, Auto Zone, etc.

http://www.bgprod.com/products/fuelair.html" onclick="window.open(this.href);return false;

http://www.bgprod.com/bgdistloc/distloc2.php" onclick="window.open(this.href);return false;

[Money pit Beretta]

There is a shop that I take my car to in their list. Thanks for the links.