Oxygen Sensor Codes Diagnostic socket A2:
EFI-153,212,425,435,436,521,533
When any of the Oxygen Sensor codes are set, the most common repair is to replace the faulty sensor, however don't over look possible causes for the sensor to have failed such as: intake leak, exhaust leak, faulty spark plugs or wires, low compression on one or more cylinders, bad gas, or failed catalytic converter. Normally when an O2 sensor code has been set, fuel trim is inhibited and possible three-way catalytic converter (TWC) damage may occur. It is always a good idea to first read the codes and then see if they return prior to replacing any sensor, however if you have access to a scan tool, the tool can tell you how many times the code has been set, and if it's more than once or twice, you can be sure it is not a glitch in the system and a repair is in order. The oxygen sensor used in the Volvo FWD 850 is a 4-wire Bosch sensor, some owners have had success at replacing the sensor with an after market (Bosch) sensor. The sensor must be soldered into the existing wiring harness, if your not handy with a soldering iron then by the Volvo replacement. Instructions for replacing the sensors can be found in the repair section of this site at bay13.
EFI-153; In order for the rear sensor to be able to monitor the three way catalytic converter and affect fuel trim, it must be at operating temperature and at the correct output voltage. Monitoring begins 3 minutes after start of the preheating. If the output voltage is different than the permitted values the code 153, Rear HO2 sensor signal missing or faulty is set.
EFI-212; After 3 minutes of monitoring if the front sensor deviates from the permitted values, code 212, front HO2S signal missing or faulty is set.
EFI-425; the rear sensor ensures the front is adjusting integrator values correctly, if all positive or negative, the rear sensor can be used to fine-tune the front sensors inputs. Tuning in only one direction by the rear sensor will set code 425, rear HO2 sensor control faulty.
EFI-435; the ECM clocks the time for the sensor to sweep from rich to lean and back to rich, excessive time will set code 435, HO2S front slow to respond.
EFI-436; the ECM monitors the compensation being made by the rear sensor and sets this code when excessive compensation is being called for. Note: the front sensor can cause this code to be set in the rear.
EFI-521 (front), EFI-522 (rear) is set if the ECM detects excessive or insufficient current in the heating circuits.
For O2 sensor replacement instructions click here.