VOLVO
ECM-25BB - Front heated oxygen sensor (HO2S), bank 2. Signal missing
System
Engine Control Module (ECM)
Volvo models this fault code applies to
Applies to the following models:
2 modelsS80 (1999–2006), XC90 (2003–)
Show models by year and engine (2)
What does this fault code mean?
Although the fault is shown as a missing signal, the ECM compares the learned correction values for the front and rear oxygen sensors on bank 2. If they differ too much from each other, the code is set. The problem could lie in either the front or the rear sensor circuit.
Possible causes
- Leakage in the exhaust system
- One of the oxygen sensors shows a richer mixture than the other
- Contact resistance or oxidation in the connectors
- Damaged oxygen sensor
What you can do
- 1Look for air leaks in the exhaust system, especially upstream of the oxygen sensors
- 2Read the code, clear it and road test the car to see whether it returns
- 3Check the oxygen sensor connectors for contact resistance and oxidation. No rust solvent spray or grease in the connectors
- 4With diagnostic equipment, compare the front and rear oxygen sensor live data while driving and see whether they differ more than expected
- 5If the connectors and exhaust are in order, one of the sensors may be damaged. Have a workshop check them
Symptoms you may notice1
- Malfunction indicator lamp (MIL) on
What the fault can affect2
- Oxygen sensor control is switched off
- Catalyst diagnostics are switched off
General guidance for car owners. Diagnosis on the car can point to a different cause.