A crucial and often overlooked step.
Intermittent and poor explanation of faults can, if your not being careful, lead you down the wrong path. When the vehicle arrives in your work bay.
Step 1) Read and record faults
Note the stored faults down on a piece of paper (or take a shot with your smart phone). This is because there are often fault codes stored in a vehicle from previous issues, like a flat battery, Also very often you will see CAN Bus related fault codes due to the sometimes frequent corruption of date by electical interference.
Step 2) Clear Fault Codes
Erase the stored fault codes form the vehicles ECU.
Step 3) Reset Fault Codes
Test drive or operate the vehicle until the fault reoccurs. Then compare reset fault codes with previously recorded faults.
Poor Fault Explanation
Imagine your customer (or wife) says “the car goes all juddery and the thingy light is on”. You go out to the car check the vehicle for fault codes and find a fault code for an air flow sensor, Tell the wife, order one and install it. Only to have the wife say “I thought you fixed it”. You drive the vehicle… Brake Shudder and the vehicle was full of fault codes from a flat battery 2 years ago.
The “Worst Case Scenario” for the fault finding technician. To be able to fix a fault with 100 percent accuracy you need to be able to witness the fault to occur and confirm the fault with tests. Any thing else is effectively guess work. Having said that there is a safety aspect to be considered. If a customer presents a car with the symptom -intermittent loss of power- And you find a fault code for the -Acceleration Pedal Position Sensor-, but are unable to duplicate the fault in any condition. It is advised to recommend the customer that the unconfirmed replacement of the sensor and servicing of the wiring (check and clean connectors) be carried out (with the potential situation of the customer trying to accelerate across and intersection and have the vehicle lose power in front of a truck). Also advise the customer that as this is an unconfirmed diagnosis that there is potential for this not solve the issue, however unlikely.