Cylinder #8 Above Knock Threshold
The PCM has detected a voltage input reading in the knock sensor circuit for a particular cylinder that is not within a predetermined manufacturer’s limit.
Code Set Parameters
Typical knock sensor circuits consist of one 5-volt signal wire. If voltage readings exceed a variation of 10-percent from the manufacturer’s reference voltage, a code will be stored in the PCM and a malfunction indicator lamp will be illuminated. Some models require multiple failure cycles in order for a malfunction indicator lamp to be illuminated.
These could include a lack of power and possibly some degree of hesitation upon acceleration, accompanied by a spark knock, and an illuminated service engine soon lamp. The service engine soon lamp may remain on constantly, illuminate and turn off intermittently, or come on for extended periods of time and then turn off for short or long periods of time.
Since the knock sensor is responsible for making the PCM aware of unexpected engine noises (primarily caused by an incorrect air/fuel mixture that leads to abnormal cylinder detonation), this code is often the reaction to another more serious problem. If other drivability codes are present, diagnose and repair those before attempting to diagnose this code. If the engine is running smoothly and quietly, and the code persists, the most likely cause is a faulty knock sensor. Frayed or broken wiring, corroded wiring and connectors, or connectors that are unplugged are also strong candidates. Some applications locate the knock sensor near the starter and the sensor connector is often broken during starter replacement.
Common causes of misdiagnosis include replacement of the knock sensor when another more serious engine malfunction is to be blamed. Make sure that the knock sensor is not simply “doing its job” before condemning it to the scrap heap.
- Many manufacturers use a knock sensor or multiple knock sensors that thread into the engine block
- Typically, V-shaped engine blocks will require ay least two knock sensors; one for each bank of cylinders
- Other automakers equip their vehicles with a knock sensor for each individual cylinder
- Remember that “Bank 1” usually refers to the engine bank that contains the number 1 cylinder and “Bank 2” is the one that does not contain the number 1 cylinder
The knock sensor is a one-wire sensor that utilizes a low-voltage signal to detect excessive noise from the engine
- If excessive internal engine noise is detected, the knock sensor will be grounded; completing the circuit and delivering a 5-volt input voltage signal to the PCM
- The PCM constantly monitors knock sensor voltage and reacts accordingly
- Once the PCM receives an abnormal signal from the knock sensor, it may adjust engine timing in order to quieten the noise or to avoid further engine damage
A scanner and a digital volt/ohmmeter will be helpful in diagnosing this code
- A manufacturer’s service manual (or equivalent) will also be necessary when attempting to diagnose this code.
Begin your diagnosis by performing a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Clear the codes and retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- After the codes are cleared, operate the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Make sure that the engine is full of clean oil
- Proceed with your diagnosis by listening for excessive engine noise
- If any other drivability codes are stored in the PCM, diagnose and repair those before attempting to diagnose this code
- It may very well be a reaction to another engine problem
- If abnormal engine noises are detected, perform a manual oil pressure test and compare your findings with manufacturer’s recommendations
- Replace worn or defective engine components as required
- Clear the codes and test drive the vehicle to make sure the repair was successful
If the engine sounds normal and no other codes are present, disconnect the electrical connector from the affected sensor and test the resistance of the sensor
- Compare your findings with the manufacturer’s specifications
- Replace the sensor as needed
- Clear the codes and test drive the vehicle to ensure that the repair was successful.
If the sensor checks out, disconnect the connector from the knock sensor and test for reference voltage
- If no reference voltage is present, inspect the circuit for burnt, shorted, or broken wiring to the knock sensor
- Engine oil that has leaked onto knock sensor wiring and connectors has been reported by technicians as causing failures more frequently than dry and clean wiring and connectors
If system wiring seems to be in proper working order, disconnect the connectors from the knock sensor and all related controllers
- Perform a continuity and resistance test on all affected circuits
- Compare your actual findings with manufacturer’s specifications and repair circuits, connectors, and/or components that do not comply
- Clear codes and test drive the vehicle to ensure that a successful repair has been performed
Some knock sensors are located on areas of the engine that make them prohibitive to testing (without removing major engine components)
- Vehicles that are equipped with such knock sensors should be diagnosed through a process of elimination that leaves the knock sensor as the last possible cause of failure
- When replacing the knock sensor, make sure that it is not threaded into a coolant passage of the engine block
- Removal of the knock sensor from a coolant passage of a hot engine could lead to severe burns upon the technician’s skin