← Back to list

1999 BMW 540i 4.4L Engine Shaking, Coolant Loss & Fault Codes P0412, P1635, P1637 - Failsafe Mode Explanation

Model: 99 BMW 540i 4.4L Fault Code: P0412 Posted: 2007-04-30 00:11

Hi everyone, I'm having a persistent issue with my 1999 BMW 540i 4.4L engine. The problem started about two weeks ago. Previously, the car was running smoothly, but one day while driving at around 65 MPH on the highway, I suddenly noticed severe engine shaking and a significant loss of power. The issue would go away for a few days before returning. I initially suspected faulty spark plugs, so I replaced them — this fixed things for just one day. After that, the symptoms returned. Now, when I start the car, the 'Engine Failsafe Program' message appears, and the engine shakes significantly. The throttle response is extremely limited: pressing the gas pedal only brings the RPMs up to 2000. I've scanned the system and received these diagnostic codes: - P0412: Secondary Air Injection System Switching Valve A Circuit Malfunction - P1635: Throttle Valve Adaptation; Lower Mechanical Stop Not Adapted (BMW) - P1637: Throttle Valve Position Control; Control Deviation (BMW) I understand that the 'Engine Failsafe Program' indicates the engine control module has entered limp-home mode — a safety feature that limits engine performance to prevent damage. I believe this is directly related to the throttle system issues. My observations: - Coolant appears to be leaking, though I've been adding small amounts daily. - The engine idles irregularly and remains in a closed throttle state even when the pedal is released. - When I disconnect the throttle position sensor (TPS), the idle instability persists. I'm not an expert on BMW systems, especially technical English, so any clear guidance on what components to inspect or replace would be greatly appreciated. Specifically: 1. What does P1635 and P1637 indicate? 2. How do these codes relate to the failsafe mode? 3. Where is the throttle position sensor located in a 1999 BMW 540i? 4. Should I replace the throttle body or clean it? Are there any fuse checks I should perform? Any advice, repair steps, or recommended resources (like official repair manuals) would be very helpful. Thank you in advance for your time and expertise.

Related fault codes

P0412P1635P1637

Comments (10)

Anonymous 2007-04-30 10:43

The 'Engine Failsafe Program' means the vehicle's ECU has entered limp-home mode — it allows limited operation to prevent engine damage. The primary suspect is likely a faulty throttle position sensor (TPS) or an issue with the electronic throttle actuator motor. The P1635 and P1637 codes suggest that the ECU cannot accurately detect the throttle's closed position, which prevents proper fuel mapping. This directly causes the failsafe behavior. While coolant loss may be a separate issue, it should still be investigated independently.

Anonymous 2007-04-30 12:07

Thank you for the clarification! I'll check the throttle sensor and adjust it as suggested.

Anonymous 2007-04-30 16:04

I'm not a BMW specialist, but modern models like the 1999 540i use electronic throttle systems — no mechanical throttle cable. The throttle body integrates both the throttle valve and sensor. Given the P1637 code, it's highly likely that the throttle actuator motor or its associated sensor is malfunctioning.

Anonymous 2007-04-30 16:37

Thanks for confirming! I already know what the throttle sensor looks like — now I'm looking for detailed instructions on how to access and test it.

Anonymous 2007-04-30 17:10

You're not serious, are you? This is a very specific technical issue that requires proper diagnostic tools and knowledge. Just guessing about components without hands-on experience isn't helpful.

Anonymous 2007-04-30 17:39

I think the original poster is being realistic. If someone plans to perform repairs independently, they should first obtain an official BMW repair manual for their vehicle model. Many throttle and sensor issues in BMWs require precise procedures — especially when resetting ECU parameters after servicing. For complex repairs like this, it's best to consult a professional or use a scan tool with proper calibration capabilities. DIY attempts without guidance can lead to further damage.

Anonymous 2007-04-30 17:58

Thank you all for your patience and detailed responses. English is not my first language — I speak it, but technical terms are challenging. I now know the general location of the throttle sensor and am actively researching online resources. I also understand that some questions may seem basic — I truly appreciate everyone's time and effort.

Anonymous 2007-05-01 08:50

To the original poster: 1. Start by locating the air filter housing. 2. Follow the ductwork to the engine bay — near the intake manifold, you'll find a black box on the side or top of the throttle body with wiring connected. That is your electronic throttle position sensor and actuator. Bonus: Your English is excellent — I've spoken to several native speakers who struggle more than you do. This helps everyone in technical discussions. Hope this assists!

Anonymous 2007-05-01 20:18

Currently, when I start the engine, it idles erratically (RPMs fluctuate) and stays at a closed throttle position. Even after disconnecting the TPS wire or manually stepping on the pedal, the idle remains unstable. I’ve taken photos of the throttle body — I’m not sure whether to replace the sensor or check fuses under the airbox first.

Anonymous 2007-05-02 07:34

Electronic throttle bodies are expensive and require specialized tools to reset minimum airflow settings after cleaning or replacement. Always clean them with a damp paper towel — avoid solvents that could damage components. If the idle issue persists after cleaning, a new throttle body or professional diagnosis is likely needed. DIY repairs without proper equipment can lead to further ECU errors.