Cooling

P00B5 — Radiator Coolant Temperature Sensor Signal Erratic

Severity: medium Drive with care, to a shop

What P00B5 means

The ECM monitors the voltage signal from the coolant temperature sensor at the radiator outlet. It normally changes smoothly as the coolant warms up or cools down. P00B5 sets when the signal drops out, jumps around or does not match actual warm-up. The usual culprits are the sensor itself, a poor connector contact or damaged wiring.

Symptoms

  • Check Engine light on with P00B5 stored
  • Temperature gauge jumps or reads wrong
  • Hard starting when cold
  • Unstable idle or stalling
  • Poor fuel economy
  • Fuel smell or smoke from the exhaust due to incorrect fueling
  • Cooling fan cycling at the wrong times

Possible causes

  1. Failed radiator coolant temperature sensor
  2. Damaged, broken or chafed wiring
  3. Corroded or loose connector terminals
  4. Leaking sensor O-ring with coolant seepage
  5. Low coolant or air pocket in the cooling system
  6. ECM fault (rare)

How to diagnose

  1. Scan for codes and freeze-frame data and look for other coolant temperature codes
  2. Check coolant level and condition with the engine cold and look for leaks
  3. Inspect the sensor, connector and harness for damage, corrosion and moisture
  4. Compare this sensor's reading with the main ECT sensor on a warm engine using a scan tool
  5. Wiggle the harness and connector with the engine running and watch for reading spikes
  6. Measure sensor resistance cold and warm and compare with specifications
  7. Test the 5 V reference, signal wire and ground back to the ECM
  8. If the sensor and wiring are good, test the ECM

Common fixes

  • Replace the radiator coolant temperature sensor and its seal
  • Repair the wiring or connector and clean the terminals
  • Top off coolant and bleed the system
  • Test and replace the ECM only if confirmed faulty

Questions and answers

What does P00B5 mean?

It means the signal from the radiator coolant temperature sensor is unstable, dropping out or jumping. The ECM cannot trust the temperature data.

Can I drive with P00B5?

Short trips with close attention to the temperature gauge are possible, but do not put off the repair. If the engine overheats, pull over and shut it off.

How do I tell a bad sensor from a bad connection?

Wiggle the connector and harness with the engine running: if readings jump, the connection or wiring is at fault. If the wiring is fine and resistance does not follow temperature, replace the sensor.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.