P0C18: Drive Motor B Position Sensor Not Learned

P0C18 is the motor B version of P0C17: the resolver offset of the second motor has not been learned and no valid earlier value is stored. GM performs the learn at key-off; it fails if motor speed exceeds 50 rpm, DC voltage stays below 192 V, phase current stays below 15 A or the learn takes longer than 1.4 s. The Volt tables rate it Type A, one trip, the 2012 two-mode trucks Type B, two trips. GM bulletin 21-NA-218 notes that when P0C17 and P0C18 set together, a programming event is needed to allow a wide position sensor learn, and that missing high voltage at the drive motor control module can be the reason. P0C4F reports a learned value beyond its limit.

Symptoms

• Malfunction indicator lamp after a repair or module programming
• Reduced propulsion or vehicle will not enter ready mode
• Motor B unavailable for driving or charging

Most likely causes

• Offset learn not completed after replacing the inverter, motor B or its resolver
• No high voltage at the drive motor control module, e.g. contactor fault or disconnected HV cable
• Control module needs reprogramming to enable a wide position sensor learn

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes with a hybrid-compatible scanner and write down the freeze frame data
2. Make sure the battery cooling air intake is clean
3. Check the 12 V auxiliary battery, which can also trigger warnings
4. Leave any high-voltage measurements to a specialized repair shop

Good to know

• It can trigger limp mode.
• It affects emissions.

What each character of P0C18 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• C: Subsystem depends on the code range
• 18: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0C18?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0C18?

The most common causes are: offset learn not completed after replacing the inverter, motor B or its resolver; no high voltage at the drive motor control module, e.g. contactor fault or disconnected HV cable; control module needs reprogramming to enable a wide position sensor learn.

Related codes

• P0C17Drive Motor A Position Sensor Not Learned
• P0C4FDrive Motor B Position Exceeded Learning Limit
• P0A45Drive Motor B Position Sensor Circuit
• P0A46Drive Motor B Position Sensor Circuit Range/Performance

Sources

• gsi.ext.gm.com: 12OBDG06 Hybrid Diagnostics
• gsi.ext.gm.com: 16OBDG01 Hybrid Diagnostics
• gsi.ext.gm.com: 18OBDG02 Hybrid Diagnostics
• static.nhtsa.gov: MC 10207913 9999

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.