P2B77: Coolant Heater B Control Circuit Low

The control circuit of coolant heater B reports a control circuit low condition. Auxiliary preheating of the coolant cannot be commanded as expected.

Most likely causes

• Control wire shorted to ground
• Heater element shorted
• Connector pins corroded - high contact resistance

What to check, in order

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

1. With the engine cold, check the coolant level; never do this when the system is hot
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Check the actual temperature in live data
4. Make sure the fan comes on when it should
5. Check the thermostat and connector
6. Measure the sensor and replace it if its readings do not make sense

Good to know

• Repair difficulty: medium

What each character of P2B77 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• B: Subsystem depends on the code range
• 77: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2B77?

Watch the temperature gauge. If it climbs or a warning appears, stop the car in a safe place and turn off the engine: overheating can cause serious damage.

What causes code P2B77?

The most common causes are: control wire shorted to ground; heater element shorted; connector pins corroded - high contact resistance.

Related codes

• P2B73Coolant Heater A Control Circuit Low

Sources

• github.com: dtc database
• en.wikipedia.org: Block heater

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.