No power at different end points of 2008 hyundai elantra

My car is not getting any power at several end points. There’s no power going to the a/ compressor, the brake light, the reverse light, and the check engine light comes on intermittently for the upstream O2 sensor. Current is running through the fuses for these end points in question so it’s not a fuse issue. I am thinking the computer might be bad, what are your thoughts?

The brake light circuit is the simplest circuit to check out. It is not tied to the computer accept for a possible status monitor of the brakes. Check the grounding from the battery to the chassis for a connection problem.

I did just that with no result.

I assume you have something to test with that indicates the presents of voltage. If so check for power getting to the brake light switch using chassis ground as your reference. If you have no voltage indication there then run a jumper wire directly to the negative battery post to use as your reference point. If there is now an indication of power then you have proved there is a grounding problem somewhere. If there still is no indication of power then I suggest you check the connection to the main distribution panel under the hood. Your previous post seemed to indicate that you have proved power is getting to the fuses there at least. There may be a problem with underside of the panel possibly. Sometimes corrosion can happen to the connections under the panel.

I have done most of what you suggested. I find it strange that I am not getting power in so many different locations. Also, check engine light goes on and off for O2 sensor but when I check it I am getting good resistance. I have cleaned of the battery terminals but nothing.

I wouldn’t worry about strange things going on with various systems until the power problem is corrected. What happened with the brake light switch testing? Did you have power to the switch when using chassis ground as your reference? If not, what about the results using the negative battery post for your ground reference?

I tested the brake lights with ground from the battery terminal and chassis. Only two of the five brake lights are getting power and lighting up. The reverse light are not coming on as well.

Since some of the brake lights are working it means that power is getting through the switch. You may have some burned out lights. I doubt the reverse lights are burned out but it may be good to check them anyways. The switch for the reverse needs to be checked for power getting to it. Without knowing what the error code is for the O2 sensor it is hard to say what is going with that. It could be wiring problem to the sensor causing the issue.

Codes for O2 sensor:

  1. P2626 O2 sensor pumping current trim circuit/open Bank 1, Sensor 1

  2. P0030 HO2S heater control circuit (Bank 1, Sensor 1)

I tested the resistance for the sensor and I got resistance. Moreover, the check engine light goes on and off (sometimes for a day or two), which gives me pause as the problem being the sensor.

I checked if power is going through the fuse going to the sensor and I got power and appropriate voltage.

Did you check the resistance clear back to the ECU for both circuits?

How do I do that? When I remove the ECU connector all I see are pins. How do I know which one is for the sensor? Also, how do one check to see if the ECU is working properly?

You need to get a manual for the wiring to refer to. Ebay is a good place to get them. Factory manuals are the best to get. They provide the best information about the car and how things are designed.

I assume the connections to the sensor are correctly made to the proper pins of the sensor. The code for the sensor element states the wire connections between the sensor and the ECU have an open wire somewhere. The heater wires have some sort of problem also, they may be open also but it may be just that voltage isn’t getting to the heater element. There should be 12 volts on the heater wires when the engine is started up or the ignition is ON.

Did you check your alternator, and battery cables for a solid connection?