303-14A Electronic Engine Controls- 4.0L: Description And Operation (page 13 of 28)

period of rich to lean and lean to rich switching monitors the response rate of the upstream sensors. 
 
Diagnosis of electrical faults is continually monitored in both the upstream and downstream sensors. This is achieved by 
checking the signal against maximum and minimum threshold, for open and short circuit conditions. 
 
Oxygen sensors must be treated with the utmost care before and during the fitting process. The sensors have ceramic 
material within them that can easily crack if dropped / banged or over-torqued. The sensors must be torqued to the 
required figure, (40-50Nm), with a calibrated torque wrench. Care should be taken not to contaminate the sensor tip when 
anti-seize compound is used on the thread.  
 
Failure Modes 

Mechanical fitting & integrity of the sensor.  
Sensor open circuit / disconnected.  
Short circuit to vehicle supply or ground.  
Lambda ratio outside operating band.  
Crossed sensors Bank A & B.  
Contamination from leaded fuel or other sources.  
Change in sensor characteristic.  
Harness damage.  
Air leak into exhaust system.  

Failure Symptoms 

Default to Open Loop fuelling for the particular cylinder bank  
High CO reading.  
Strong smell of H2S (rotten eggs) till default condition.  
Excess Emissions.  

It is possible to fit front and rear sensors in their opposite location. However the harness connections are of different 
gender and colour to ensure that the senors cannot be incorrectly connected. In addition to this the upstream sensors 
have two holes in the sensor tip, whereas the down stream sensors have four holes in the sensor tip for the gas to pass 
through.  
 

KNOCK SENSORS 

 
The ECM uses active knock control, which serves to prevent engine damaging pre-ignition or detonation under all 
operating conditions enabling the engine to operate without additional safety margins. For the ECM to be able to 
determine the point at which a cylinder is pre-detonating, 2 piezo-ceramic sensors are mounted on the engine block. Each 
sensor monitors engine knock by converting the engine block noise into a suitable electrical signal, which is then 
transmitted back to the ECM via a twisted pair cable. The signal is then processed within the ECM to identify the data that 
characterises knocking. 
 
This information is compared to known signal profiles to determine whether knock is present. If so, the closed loop control 
system then retards the ignition on that cylinder, for a number of cycles, after which it gradually moves back towards its 
original setting. 
 
Failure Symptoms  
 
The following describes the failure symptoms of the knock sensors: