The P0890 OBDII code in your 2016 Subaru Forester indicates a problem with the Transmission Control Module (TCM) power supply circuit, specifically pointing to a potential issue with the “Self-Cut-Off Relay”. This code can prevent your vehicle from shifting gears properly, leading to significant driveability problems. This guide provides a step-by-step diagnostic procedure to help you pinpoint and fix the P0890 fault.
This diagnostic process is designed to be thorough and systematic, ensuring you can accurately identify the source of the P0890 code. It involves checking the wiring harness, power supply to the relay, and the relay itself. By following these steps, you can determine whether the issue lies in a faulty relay, a wiring problem, or potentially a more complex TCM issue.
Step-by-Step Diagnostic Procedure for P0890 Code
Before starting, ensure your vehicle is safely parked and the ignition is turned off. You will need a multimeter to perform the following tests.
1. Check Harness for Open or Short Circuit (Resistance Check)
This step involves checking the continuity and resistance of the wiring between the TCM and the self-cut relay.
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Disconnect Connectors: Disconnect the TCM connectors and the self-cut relay connector. This is crucial for safe and accurate measurements.
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Measure Resistance: Using a multimeter set to measure resistance (Ohms), measure the resistance between the specified terminals on the TCM connector and the self-cut relay connector. Refer to the table below for terminal numbers based on your Subaru Forester model (Left Hand Drive or Right Hand Drive).
Left Hand Drive Model:
- (B55) No. 20 – (B220) No. 16
- (B55) No. 2 – (B220) No. 15
- (B55) No. 11 – (B220) No. 15
- (B55) No. 22 – (B220) No. 15
Right Hand Drive Model:
- (B55) No. 20 – (B220) No. 30
- (B55) No. 2 – (B220) No. 29
- (B55) No. 11 – (B220) No. 29
- (B55) No. 22 – (B220) No. 29
Expected Result: The resistance should be less than 1 ohm.
If the resistance is higher than 1 ohm, there is an open circuit in the wiring harness. Repair the open circuit before proceeding.
2. Check Harness for Short to Ground (Resistance Check)
This step verifies if there is an unintended short circuit from the TCM connector to the vehicle body ground.
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Measure Resistance: With the TCM connectors still disconnected, measure the resistance between each of the specified terminals on the TCM connector (B55) and a known good body ground point.
- (B55) No. 20 – Body Ground
- (B55) No. 2 – Body Ground
- (B55) No. 11 – Body Ground
- (B55) No. 22 – Body Ground
Expected Result: The resistance value should be 1 MΩ (Megaohm) or more.
If the resistance is less than 1 MΩ, there is a short circuit to ground in the wiring harness. Locate and repair the short circuit.
3. Check Relay Power Supply Circuit (Voltage Check)
This step checks if the self-cut relay is receiving power.
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Measure Voltage: Disconnect the self-cut relay. Set your multimeter to measure DC Voltage. Measure the voltage between the specified terminals on the self-cut relay connector (B220) and a body ground point.
Left Hand Drive Model:
- (B220) No. 14 (+) – Body Ground (-)
- (B220) No. 17 (+) – Body Ground (-)
Right Hand Drive Model:
- (B220) No. 28 (+) – Body Ground (-)
- (B220) No. 31 (+) – Body Ground (-)
Expected Result: The voltage should be 10V or more.
If the voltage is less than 10V, there is an issue with the power supply circuit. Repair any open or short circuits in the wiring harness supplying power to the relay.
4. Check Self-Cut Relay (Resistance Check – Coil)
This step tests the internal coil resistance of the self-cut relay.
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Measure Resistance: Using your multimeter set to measure resistance, measure the resistance between the specified terminals on the self-cut relay.
Left Hand Drive Model:
- (B220) No. 16 – (B220) No. 17
Right Hand Drive Model:
- (B220) No. 30 – (B220) No. 31
Expected Result: You should measure a specific resistance value, typically in the range of hundreds of Ohms (refer to the service manual for the exact specification for your relay). A very low or very high resistance indicates a faulty relay coil.
5. Check Self-Cut Relay (Resistance Check – Switch)
This step tests the switch contacts of the self-cut relay.
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Measure Resistance: Measure the resistance between the other set of terminals on the self-cut relay.
Left Hand Drive Model:
- (B220) No. 14 – (B220) No. 15
Right Hand Drive Model:
- (B220) No. 28 – (B220) No. 29
Expected Result: The resistance value should be very high, ideally 1 MΩ or more, indicating an open switch when the relay is not energized.
If the resistance is low, the relay contacts may be stuck closed, and the relay should be replaced.
If all previous steps pass and the relay tests indicate a fault, replace the self-cut relay with a new, genuine Subaru part.
6. TCM Input Signal Check (Voltage Monitoring)
After performing the wiring and relay checks, reconnect all connectors and use a diagnostic tool like the Subaru Select Monitor to read the “Control module voltage” data.
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Connect and Monitor: Reconnect the TCM connectors and the self-cut relay. Turn the ignition to the “ON” position.
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Read Data: Using the Subaru Select Monitor, access and read the “Control module voltage” data parameter.
Expected Result: The “Control module voltage” should be 10V or more.
If the voltage is 10V or more, the electrical system is currently functioning normally. The issue might be intermittent or related to poor connections. Proceed to step 7.
7. Check for Poor Contact Reliability
If the “Control module voltage” is normal, the problem might be due to poor contact in the self-cut relay circuit or other connectors.
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Inspect Connectors: Carefully inspect all connectors in the self-cut relay circuit and TCM power supply for signs of corrosion, loose terminals, or damage. Clean and reseat connectors as necessary.
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Check Relay Socket: Inspect the relay socket for any damage or corrosion.
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Test Drive: After checking connections, clear the P0890 code using an OBDII scanner and perform a test drive to see if the code returns.
Conclusion
By following these diagnostic steps, you should be able to effectively troubleshoot and resolve the P0890 code on your 2016 Subaru Forester. In many cases, the issue stems from a faulty self-cut relay or wiring harness problems. If all checks pass and the code persists, or if you are not comfortable performing these tests, it is recommended to consult a qualified automotive technician for further diagnosis and repair. Remember to always clear the ECM memory after completing any repairs related to the TCM power supply circuit.
