Is your Mini Countryman or BMW showing a check engine light with code P0036 (or the specific BMW code 12B304)? While the car might seem to drive normally, this "Oxygen Sensor Heater" fault can lead to poor fuel economy and failed emissions tests.
In this guide, based on a diagnostic walkthrough by SA Diagnostic's, we’ll explain where this sensor is hiding, why "mapping it out" is a bad idea, and how to verify the fix using the iCarsoft CR ULTRA P.
The silver connector usually sits on top of the engine, but the wire traces all the way down to the exhaust.
Understanding P0036: The "Heater" Fault
Modern Oxygen Sensors need to be hot (over 600°F) to read exhaust gases accurately. To get them working quickly on cold starts, they have an internal electric heater.
- The Code: P0036 refers specifically to the Heater Circuit of Sensor 2 (the one after the catalytic converter).
- The Cause: Usually, the heating element inside the sensor burns out over time, just like a lightbulb filament.
Technical Resource: Why Oxygen Sensors Have Heaters (NGK/NTK Technical).
Video Tutorial: Tracing the Wiring
See exactly where the connector is located and how the wire runs down to the exhaust:
Repair Options: Fix vs. Hack
1. The Proper Fix: Replace with OEM
The technician advises: "It's best to replace it with an OEM sensor rather than aftermarket."
- Location: As shown in the video, follow the silver wiring sheath down the back of the engine. The sensor is screwed into the exhaust pipe after the catalytic converter.
- Difficulty: Access can be tight. You may need an "O2 Sensor Socket" and extension bars.
- Parts: Stick to brands like Bosch or NTK. Cheap unbranded sensors often have the wrong heater resistance, causing the code to return immediately.
2. The "Map Out" Option (Not Recommended)
The video mentions you can "remap it and map it out." This involves reprogramming the ECU to ignore the rear O2 sensor.
Warning: While this clears the light, it is illegal in the UK, Europe, and many US states. It will result in an immediate failure during annual inspections (MOT/Smog Check). We always recommend fixing the actual fault.
Use Live Data to verify the heater circuit before and after replacement.
Diagnostic Tip: Use Your Scanner
Before you struggle to remove the rusty sensor, confirm the fault with your iCarsoft CR ULTRA P:
- Go to Live Data > O2 Sensor Bank 1 Sensor 2.
- Look for Heater Current or Heater Status.
- If the status is "Active" but the Current is 0.00 Amps, the internal circuit is definitely broken (Open Circuit).
Once replaced, use the tool to Clear Fault Codes. The check engine light should turn off and stay off.
Frequently Asked Questions (FAQ)
Can P0036 cause loss of power?
Indirectly, yes. If the ECU detects an O2 sensor heater failure, it may stay in "Open Loop" mode longer or default to a safe fuel map, which can feel like a slight loss of power or reduced fuel efficiency.
How much does a new sensor cost?
As mentioned in the video, an OEM quality sensor typically costs around £80-£100 ($100-$130). Labor costs will vary depending on how rusted the sensor is into the exhaust.
Does CR ULTRA P work on BMW 12B304 code?
Yes. The CR ULTRA P reads both standard OBD2 codes (P0036) and the specific BMW manufacturer codes (12B304), giving you a more accurate diagnosis than generic readers.
Pass Your Emissions Test with Confidence.
Don't let a heater fault fail your inspection. Diagnose and clear codes with the iCarsoft CR ULTRA P.
