DDEC III/IV Single ECM Troubleshooting

Section 18.4 Troubleshooting Flash Code 18, Relative Humidity Sensor Circuit Failed Low

Section 18.4
Troubleshooting Flash Code 18, Relative Humidity Sensor Circuit Failed Low

Follow these steps to troubleshoot Code 18, relative humidity sensor circuit failed low.‪

Section 18.4.1
Code Check

Perform the following steps to check for codes:‪

  1. Start and run engine for 6 minutes, or until CEL is illuminated.
  2. Plug in DDR and check for codes.
    1. If active code p 354/4 and any other codes are logged, troubleshoot the other codes first.
    2. If active code p 354/4 is logged, go to section "18.4.2 Check Connectors" .
    3. If no codes are logged, return the vehicle to service.

Section 18.4.2
Check Connectors

Perform these steps to check connectors:‪

Note: This sensor is wired via the 10–pin connector. See the wiring diagram; see Figure "Connectors" .

Click to see this graphic in a separate window

Figure 1. Connectors

  1. Check connectors for corrosion.
  2. Disconnect the connector at the RH sensor.
  3. Measure the resistance between cavities 2 and 1; see Figure "Connectors" .
    1. If measured resistance is < 5 Ω, #976 is shorted to #452. Repair the short and test.
    2. If measured resistance is > 5 Ω, go to the next step.

Section 18.4.3
Check for Open

Perform the following steps to check for open:‪

  1. Insert a jumper wire between cavity 2 and 3 of the RH sensor connector. See Figure "Connectors" .
  2. Unplug the engine side 30–pin connector.
  3. Measure the resistance between L2 and W1; see Figure "Connectors" .
    1. If measured resistance is less than 5 Ω, replace the sensor and test.
    2. If measured resistance is greater than 5 Ω, either the signal #976 or supply #416 is open. Repair the open and test.


DDEC III/IV Single ECM Troubleshooting Guide - 6SE497
Generated on 10-13-2008

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