# 99 sentra rough idle, bad mileage and p0138 code



## kiilez (Mar 15, 2014)

My girlfriend has a 99 se sentra with the sr20de in it. She just recently bought the car and I ended up having to put a new radiator in it. Right after we replaced the radiator the cel came on. I checked the codes and got a p0138 high voltage from o2 sensor 2. It hasnt been getting very great mileage and it idles rough. I replaced the o2 sensor and checked fuel pressure. It was making 40psi. 

Everything on the car looks brand new. Plus and wires are new, sensors are all new, brand new oil, and now a brand new radiator. Ive been trying to fix it this problem and havent had any luck. Any insight on this would be appreciated. I have been researching the forum, but havent found anything.


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## kiilez (Mar 15, 2014)

Anyone have any ideas?


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## worthirt (May 21, 2013)

where did you get the sensor? Quality control from some makers really sucks, you may have gotten a bad one. You may want to check the wires and connections again also. You may find a bad one.


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## kiilez (Mar 15, 2014)

Would that cause bad gas mileage?


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## worthirt (May 21, 2013)

Actually, yes. I had a 98 Jetta with a bad rear O2 sensor. It was getting 18MPG bad and around 24 after I put a good one in it. It just checks the CAT but if the ECU is thinking the CAT is bad, it keeps the motor at high idle. Ran fine, just used more fuel because of it.


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## kiilez (Mar 15, 2014)

Would this cause the rough idle as well? It's also having trouble starting after the engine has completely cooled off.


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## worthirt (May 21, 2013)

Usually not, the rear O2 sensor(s) don't have influence on idle. That's probably something else, but let's get one at a time. Let me look up the test for the sensor, the sensor may be good, the cat itself may actually be bad. Have the floorboards gotten obscenely hot? That is one indication of a bad CAT.


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## kiilez (Mar 15, 2014)

Actually no. It runs great other than idle and the MPGs arent as bad as I thought. I went ahead and replaced the old o2 universal sensor with a new oem one just rule that out. Ive checked the aac and egr. I havent had a chance to do the injectors or clean the MAF. It aldo has an issue starting when its cold and it idles really low when you start after it not running. Im thinking that might be the air regulator.


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## azkicker0027 (May 31, 2002)

have you also looked at the pre-catalytic oxygen sensor? this one actually controls your air/fuel mixture. The post-cat sensor is mainly for monitoring the catalyst efficiency and emissions. If you have a scanner, try to look at the voltage readings of both sensors. If they're close to each other, you may have a bad catalytic converter. Post cat sensors should have slower movement/voltage change than the pre-cat, and slightly lesser voltage readings.


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## kiilez (Mar 15, 2014)

I actually do have a scan tool and the post cat is working properly as is the precat o2 sensor. I thought maybe it could be idle controls that are causing the rough idle


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