It’s a perplexing situation for any car owner when their vehicle refuses to start unless an OBD2 scanner is plugged in. This issue, highlighted by a user working on a 2003 Honda Element with a KA24 engine, points to a deeper underlying problem within the car’s electrical or computer systems. The owner initially encountered a no-start condition after moving the vehicle into the shop, accompanied by erratic behavior from the instrument cluster. Intriguingly, connecting an OBD2 scanner allowed the engine to start, only to stall immediately upon disconnection. A P0606 code (ECM Processor Failure) was retrieved, and even after replacing the ECM, the problem persisted. This indicates that the ECM itself might not be the root cause, or there could be additional factors at play. Let’s delve deeper into potential causes and troubleshooting steps when your OBD2 scanner only works when the car is on.
Decoding the Symptoms: Why Does Your Car Start with the OBD2 Scanner Connected?
The symptoms described are crucial for diagnosis. Let’s break them down:
- No start without OBD2 scanner: The engine cranks but doesn’t fire, indicating a lack of spark, fuel, or proper engine management.
- Erratic instrument cluster: Jumping RPM and speedometer needles suggest electrical instability or issues within the car’s communication network.
- Starts with OBD2 scanner connected: This is the key symptom. The act of plugging in the scanner temporarily resolves the starting issue.
- Stalls when scanner disconnected: This reinforces the dependency on the OBD2 scanner for the engine to run.
- P0606 code (ECM Processor Failure): While initially pointing to ECM issues, the persistence of the problem after replacement suggests this code might be a symptom rather than the primary cause.
- Low ELD Voltage: The mentioned low ELD (Electrical Load Detector) voltage might be a red herring but could indicate a broader electrical system anomaly.
These symptoms collectively point towards a problem that is somehow being bypassed or compensated for when the OBD2 scanner is connected.
Potential Causes and Troubleshooting Steps
Several factors could explain why your car starts only when an OBD2 scanner is plugged in. Here are some potential areas to investigate:
1. Grounding Issues
Poor grounding is a common culprit in automotive electrical problems. A loose or corroded ground connection can disrupt the flow of electricity and cause various malfunctions.
- Check Main Ground Points: As the original poster mentioned cleaning and regrounding under the hood, it’s vital to ensure all main engine and chassis ground straps are clean, tight, and free of corrosion. Pay particular attention to the ground points for the ECM and instrument cluster.
- Inspect Battery Terminals and Cables: Ensure battery terminals are clean and tightly connected. Check battery cables for damage or corrosion, especially at the ground connection to the chassis.
2. Power Supply Problems
The ECM and other critical engine management components require a stable and consistent power supply. Voltage drops or fluctuations can lead to starting issues.
- Battery Voltage Test: Verify the battery voltage is within the normal range (around 12.6V at rest and above 14V when charging). A weak battery can cause erratic electrical behavior.
- Check Fuses and Relays: Although the original poster checked fuses, double-check all fuses related to the ECM, ignition, and fuel systems in both under-hood and under-dash fuse boxes. Test relays for proper function, especially the main relay and fuel pump relay.
- Ignition Switch: A faulty ignition switch might not be providing consistent power to all circuits in the starting and running positions. Test the ignition switch for proper voltage output in all key positions.
3. ECM and Wiring Harness Issues
While the ECM was replaced, there could still be wiring harness problems affecting the ECM or other sensors.
- OBD2 Port Wiring: The fact that plugging in the scanner resolves the issue suggests a possible problem with the OBD2 port wiring itself. Inspect the OBD2 port connector and wiring for damage, corrosion, or loose connections. Specifically, check the power and ground pins of the OBD2 port. It’s possible a broken wire or poor connection is being temporarily bridged when the scanner is connected, completing a necessary circuit.
- ECM Wiring Harness Inspection: Carefully inspect the ECM wiring harness for any signs of damage, chafing, or rodent intrusion. Pay attention to wires that might be related to power and ground for the ECM.
- CAN Bus Communication: The erratic instrument cluster could indicate a problem with the CAN (Controller Area Network) bus communication. The OBD2 scanner might be somehow stabilizing the CAN bus when connected. Investigate CAN bus wiring and connections.
4. Immobilizer System
Although less likely, a malfunctioning immobilizer system could theoretically be bypassed or temporarily reset by connecting an OBD2 scanner in some rare scenarios.
- Immobilizer System Check: If the vehicle has an immobilizer system, ensure it is functioning correctly. Look for any immobilizer warning lights or indicators on the dashboard. Try starting with a different key if available.
The OBD2 Scanner as a Temporary Fix: What’s Really Happening?
The key to understanding this problem lies in why the OBD2 scanner allows the car to start. Here are a few theories:
- Ground Completion: The OBD2 scanner, when plugged in, might be providing an alternative ground path, bypassing a faulty ground connection in the car’s wiring.
- Power Supply Bridge: Less likely, but the scanner might be supplying a small amount of power that is just enough to activate a relay or circuit that is failing due to a voltage drop.
- CAN Bus Stabilization: The scanner’s presence on the CAN bus might be stabilizing communication if there’s a signal integrity issue or termination problem.
Conclusion: Systematic Troubleshooting is Key
Diagnosing why an OBD2 scanner only works when the car is on requires a systematic approach. Start with the basics: check grounds, power supply, and fuses. Then, move towards more specific components like the OBD2 port wiring, ECM harness, and potentially the immobilizer system.
While the P0606 code initially pointed towards an ECM failure, the continued issue after replacement indicates a more nuanced problem. The act of plugging in the OBD2 scanner is providing a crucial clue. Focus on what electrical changes or circuit completions might be occurring when the scanner is connected and absent when it is disconnected.
If you are not comfortable performing these electrical diagnostics, it is highly recommended to seek professional help from a qualified automotive technician. They will have the tools and expertise to accurately diagnose and repair this unusual issue.
