What P0AA8 means
The two isolation sensor readings, positive rail to ground and negative rail to ground, should together equal the total pack voltage.
How the engine computer detects it
P0AA8 is the plausibility check of that relation. GM's 2016 and 2019 tables fail it when the sum of both readings differs from the measured battery voltage by more than 5 V in 40 of 50 samples, with the bias switch open, Type B, two trips. GM's 2013 table compares mid-pack sensors with the motor controller's voltage reading and allows 15 V. Mitsubishi groups P0AA8 with P0AA7 and P0AA9 as leak detection circuit faults, which on 2018 Outlander PHEVs were set falsely by noise. Unlike P0AA9 or P0AAA, the signal stays in range but is implausible; P0AA6 is a real insulation loss.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine and hybrid or EV system warnings
- Reduced power or speed limit until restart
- Isolation monitoring unavailable, so related faults may go unreported
Common causes
- Isolation measuring amplifier or reference in the battery controller out of tolerance Most likely
- Pack voltage sense lead fault making the comparison value wrong Possible
- Controller software misreading electrical noise (update available on some models) Possible
How to diagnose P0AA8
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: hv battery voltage isolation sensor, wiring harness, bms.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- A
- Hybrid and electric propulsion
- A8
- Specific fault
Questions drivers ask
Can I drive with code P0AA8?
Have it checked before you drive further. An isolation fault means the high-voltage system may be leaking voltage to the car's body. Many hybrids and EVs limit or shut down the drive system. Never touch orange high-voltage cables, and leave the repair to a shop trained on hybrids.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.