P0AA9 on Mitsubishi: How to Diagnose & Fix
Hybrid Battery Voltage Isolation Sensor Circuit Low
1 Mitsubishi model · 1 documented issue · 1 critical
P0AA9 on Mitsubishi by model: symptoms, diagnosis and fix
On 2018 Outlander PHEVs built September 14, 2017 through August 7, 2018, inappropriate programming in the sub-Battery Management Unit lets electrical noise from the drive selector, brake application or air-conditioner operation be misread as a high-voltage isolation (leak) fault. The car then lights Engine Malfunction, EV System Service Required and RBS warnings together, stores P0AA7, P0AA8 and/or P0AA9 (Leak Sensor Detection Circuit), cuts power and restricts speed to 37 mph (60 km/h). Mitsubishi issued Customer Satisfaction Campaign SC-18-001 in October 2018 rather than a safety recall, so it does not appear in a NHTSA VIN recall search.
Common Symptoms
- Engine Malfunction, EV System Service Required and RBS warnings at once
- Speed limited to about 37 mph
- Fault triggers when shifting, braking or switching on A/C
- Codes P0AA7, P0AA8, P0AA9
How to Fix
For an eligible 2018 Outlander PHEV, Mitsubishi campaign SC-18-001 updates the sub-BMU software free; the owner letter estimates about 0.5 hour, with additional dealer scheduling time possible. Owners who paid for this specific condition can submit original repair and payment documents to Mitsubishi Customer Relations for reimbursement consideration. If warnings recur, have a qualified Mitsubishi technician diagnose the stored codes using the applicable service procedure; recurrence alone does not establish a particular high-voltage fault.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (3)
P0AA9 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0AA9 mean on a Mitsubishi?
P0AA9 stands for "Hybrid Battery Voltage Isolation Sensor Circuit Low." The HV isolation sensor circuit voltage is below the expected threshold (short to ground or sensor failure). On Mitsubishi it is documented across 1 model: Outlander.
What are the most common causes of P0AA9?
The most common causes of P0AA9 are: Signal wire shorted to ground, Failed isolation sensor, Connector/wiring fault, Control module fault. The specific cause varies by vehicle.
How much does it cost to fix P0AA9 on a Mitsubishi?
Repair costs vary widely with the root cause and Mitsubishi model; each issue on this page carries its own estimate where one is published.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.