P2B77: Coolant Heater B Control Circuit Low

What it means

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

At a glance

• Usually lights the check-engine light

How it works

Coolant heater B warms coolant electrically when the control module requests it. The module monitors heater B's control circuit and sets P2B77 when its signal is lower than expected, which can indicate a short to ground, a shorted element, or a poor connector connection.

Common causes

Most likely first.

1. Control wire shorted to ground
2. Heater element shorted
3. Connector pins corroded - high contact resistance

Symptoms

• Check-engine light
• Coolant or engine may take longer to warm up
• Cabin heat may take longer to become warm

How to diagnose it

1. Use freeze-frame and live data to confirm the fault conditions and whether heater B was commanded on.
2. Inspect heater B's wiring and connector for a control wire rubbed through to ground, corrosion, or damaged pins.
3. With power isolated as specified by the manufacturer, check the heater B control wire for an unintended path to ground.
4. Test heater B's element and connector contact condition using the manufacturer's procedure and specifications.
5. If no short or connection fault is found, check the control module output and circuit readings against the manufacturer's procedure.

Common fixes

• Repair a heater B control wire shorted to ground.
• Clean or repair corroded or loose heater B connector pins.
• Replace heater B if its element tests shorted.
• Repair the circuit connection if testing identifies excessive contact resistance.

Can I keep driving?

Driving is generally possible if the engine temperature remains normal, but heater B may not preheat the coolant and cabin heat may take longer. Avoid depending on prompt heat in cold conditions, and stop if the engine overheats.