Questions and Answers

Nissan MAP Or Knock Sensor Q. Vince, I have a 1996 Nissan Maxima, V-6, Automatic, 87,000 miles on it. Lately I notice the check engine light is on. I immediately went to Auto Zone to get it a free check up. They hook up a diagnostic device and found out that there is a circuit malfunction on the knock sensor and MAP sensor.

My question is should I change it with new sensors and what's going to happen to my car if I don't?

Thank you very much for your time.
Karl

A. Whenever I see two unrelated problems (I could have used the actual code numbers) the next thing I do is clear all the codes and drive the vehicle. When the MIL comes back on I recheck the codes and 99 times out of 100, one of the codes is gone. If both come back then it's usually a wiring problem.

I've seen MAP sensors go bad, but I don't think I ever remember seeing a Knock Sensor go bad. To check the MAP sensor:

1. Remove MAP sensor with its harness connector connected.
2. Remove hose from MAP sensor.
3. Turn ignition switch "ON" and check output voltage between terminal (2) and engine ground. The voltage should be 3.2 to 4.8 V.
4. Use pump to apply vacuum of -26.7 kPa (-200 mmHg, -7.87 inHg, -3.87 psi) to MAP sensor as shown in figure and check the output voltage. The voltage should be 1.0 to 1.4 V lower than the value measured in step 3.
5. If NG, replace MAP sensor.

To test the Knock Sensor:

1. Disconnect knock sensor harness connector.
2. Check resistance between terminal (2) and ground. Approximately 500 - 620 k Ohms at 25° C (77° F)
   • It is necessary to use an ohmmeter which can measure more than 10 M Ohms.

If they both check out, then you have a wiring problem.