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Discussion Starter #1
Hey Guys,

I popped a Code and it came up as:

Code 25: Pre-Cat 02 Sensor Heater Control Cyl #1-4

I searched through the Peak code knowledgebase and the DIY and it lead me to check the wiring in the DME and at the link between the sensor and the plug/connection. The connections look fine to me.

My question is: if it's the heater control for the sensor, should I still just buy a new 02 sensor? would that actually fix it? I THINK the heater wires are inside the sensor to heat it up, but I don't want to buy the part just to have the code pop if it's actually something else.

I know this may seem super simple, but the simplicity is exactly why it's got me stumped as to whether to drop the $100 to buy 2 sensors at once (might as well fix both at the same time if this is actually the cause). I haven't actually seen this FULLY addressed anywhere in the forums/google. Each thread I read leads to a different fix for something else.

Just looking for piece/peace of mind before I drop the $100 on amazon for a set.

Thx!
 

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Yes, the heater is built into the O2 sensor.

There are 4 wires going into the sensor, two for the sensor and 2 for the heater. The wiring diagram below shows the heater circuit on the left, getting constant 12V from fuse F3 and being switched to ground by the DME when heat is required.

If you have a good digital multimeter, you can confirm it is bad. If you disconnect the connector at the sensor, and measure the resistance between pins 1 and 2 on the sensor connector (you may have to get the pin numbers from the harness-side connector), you should read a few ohms. If you read 0 ohms or OL the heater has failed. You can confirm from the bank 2 O2 sensor what the resistance is for a good sensor.

O2 sensor.JPG
 

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Discussion Starter #3
Yes, the heater is built into the O2 sensor.

There are 4 wires going into the sensor, two for the sensor and 2 for the heater. The wiring diagram below shows the heater circuit on the left, getting constant 12V from fuse F3 and being switched to ground by the DME when heat is required.

If you have a good digital multimeter, you can confirm it is bad. If you disconnect the connector at the sensor, and measure the resistance between pins 1 and 2 on the sensor connector (you may have to get the pin numbers from the harness-side connector), you should read a few ohms. If you read 0 ohms or OL the heater has failed. You can confirm from the bank 2 O2 sensor what the resistance is for a good sensor.

View attachment 409826
So I bought two o2 sensors, replaced both pre-cat o2s and i'm still getting the code 25 - pre-cat bank 1 heater sensor error...

Any ideas? I pulled it back out and re-installed it, did nothing, couldn't tell anything wrong with the wiring, or from the DME...

LMK your thoughts.
 

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So you have a problem outside the sensor, i.e wiring, connectors, or a DME problem. If you want to absolutely confirm that, swap the sensors from side to side to confirm the code stays on bank 1.

The sensor wiring passes quite close to the exhaust. Check for burnt wiring, usually due to the wiring coming out of its supports.

Had any pieces fly off the giubo? That can tear up the sensor wiring.

Do you have a digital multimeter and are you comfortable with using it?
 

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Discussion Starter #5
So you have a problem outside the sensor, i.e wiring, connectors, or a DME problem. If you want to absolutely confirm that, swap the sensors from side to side to confirm the code stays on bank 1.

The sensor wiring passes quite close to the exhaust. Check for burnt wiring, usually due to the wiring coming out of its supports.

Had any pieces fly off the giubo? That can tear up the sensor wiring.

Do you have a digital multimeter and are you comfortable with using it?
I will try the sensor swap - don't have a multimeter yet - I think I'll swing by the hardware store tonight.

Thanks!
 

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If that fuse is bad, all the new sensors in the world will not turn off that CEL!

;)

Check the fuse. You really need a multimeter/continuity tester....

(Question- is there one fuse per sensor? One per bank? I cannot get at the schematic right now... thx)
 

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If that fuse is bad, all the new sensors in the world will not turn off that CEL!

;)

Check the fuse. You really need a multimeter/continuity tester....

(Question- is there one fuse per sensor? One per bank? I cannot get at the schematic right now... thx)
ard, fuse F3 in the schematic I posted above also supplies the bank 2 heater.
 

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Discussion Starter #9
Also get one of those cheap test lights with a filament bulb, not the LED one. They're a great tool for circuit testing.
Okay - I will get ALL THE THINGS :)

I think I have time to go to the HW store tonight or tomorrow. I'll report back.

Thanks for they help so far guys!
 
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