01 How Many Gears Are Best for Drag Racing? • 02 Automotive Abbreviations: A Guide for Car Enthusiasts • 03 Why Your Car Revs High But Doesn’t Go Fast • 04 Alternator Versus Battery Which One Actually Keeps Your Car… • 05 Important Safety Recall: Ford Recalls 1 Million Vehicles fo… • 06 Engineers Reveal Why Most Car Doors Are Designed To Rip Apa… • 07 The 9 Silent Signs of a Failing Oxygen Sensor • 08 C0040 and C0041: Right Front Wheel Speed Circuit Malfunction • 09 What is an ABS Pump and How Does It Work? • 10 Maximizing Profit Margins in Auto Service Shop •

DTC codes / P0Exx · Hybrid propulsion / P0E77

P0E77Coolant Bypass Valve B Control Circuit/Open

P0E77 indicates the powertrain control module has detected an open circuit or excessive resistance on the control circuit for the coolant bypass valve B, which is used to route coolant away from the heater core or radiator during warm-up. The computer commands the valve to move but does not see the expected electrical feedback, so it sets this code and may default the valve to a safe position.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Slower than normal engine warm-up or heater performance that is weak until the engine is fully warm
  • Coolant temperature gauge that rises more slowly than usual or fluctuates slightly during warm-up
  • A slight decrease in fuel economy during cold-start driving because the engine stays in warm-up enrichment longer
  • In some vehicles, a faint clicking or buzzing from the valve area when the ignition is turned on, indicating the PCM is attempting to actuate the valve
  • Rarely, a stored pending code that reappears after clearing without any drivability change

Common causes, most likely first

  1. Broken, chafed, or corroded wire in the control circuit between the powertrain control module and the coolant bypass valve B
  2. Failed coolant bypass valve B actuator, such as a burned-out solenoid coil or stuck internal mechanism
  3. Loose or corroded electrical connector at the valve, often caused by coolant seepage or heat cycling
  4. Open fuse or relay in the valve's power supply circuit, though this usually affects multiple circuits
  5. Powertrain control module driver circuit failure, which is rare but possible after a shorted valve solenoid
  6. Aftermarket or incorrect valve installed previously that does not match the vehicle's electrical characteristics

How to diagnose and fix it

  1. Clear the code and perform a road test with a scan tool to confirm whether P0E77 returns as current or pending, and record any freeze-frame data.
  2. Visually inspect the coolant bypass valve B connector and the harness routing for chafing, coolant residue, or loose pins, then disconnect and check for corrosion or pushed-out terminals.
  3. Measure resistance across the valve solenoid with a multimeter and compare to the manufacturer's specification; an open or out-of-range reading points to a failed valve.
  4. Backprobe the control circuit at the valve and at the powertrain control module to check for continuity and excessive voltage drop while wiggling the harness.
  5. Command the valve on and off with a bidirectional scan tool and listen for actuation or watch for a change in live data; no response confirms a circuit or valve fault.
  6. If the circuit tests good, substitute a known-good valve or test the original valve on a bench to isolate the actuator from the wiring.
  7. Repair or replace the faulty harness section, connector, or valve as needed, then clear codes and verify the repair with a complete drive cycle.

What the bypass valve code really means

  • A coolant bypass valve B control circuit open code can set even when the valve mechanically moves freely, because the PCM monitors electrical current and not physical position on many designs.
  • Technicians often replace the valve first and later find a corroded connector pin or a wire rubbed through near the exhaust manifold, so always inspect the harness before condemning the part.
  • On some vehicles, the same valve is shared with the heater control circuit, so a fault in the heater plumbing or a restricted heater core can cause the valve to be commanded in a way that sets this code indirectly.
  • A weak or failing battery or charging system can cause intermittent open-circuit codes on actuator circuits because the PCM driver may not have enough voltage to keep the circuit stable during cranking.
  • This code does not always mean the valve is stuck open or closed; it specifically flags an electrical open, so a valve that is mechanically stuck but electrically intact will usually set a different code.

Can I keep driving with P0E77?

You can usually continue driving with P0E77 because the engine will still warm up, though heater performance may be reduced and fuel economy can suffer slightly during warm-up. Avoid long periods of idling in cold weather and have the circuit inspected soon, especially if the check engine light flashes or coolant temperature behaves erratically.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage