4Runner 4.0L

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Knock sensors No. 1 and 2 are located below intake manifold on cylinder block. See Fig 1. Manufacturer recommends removing knock sensor from cylinder block before testing. See KNOCK SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL & INSTALLATION - V6 & V8.
Using an ohmmeter, measure the resistance between the knock sensor terminals. Resistance should be 120-280 k/ohms at 68°F (20°C). See Fig 2. If the resistance is not as specified, replace the knock sensor. See KNOCK SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL & INSTALLATION - V6 & V8.

Fig 1: Locating Knock Sensors (4Runner 4.0L)

Fig 2: Testing Knock Sensor (4Runner 4.0L)