My mother in-law has a 94 Chrysler LHS, 3.5 V6, auto, 66k. Yesterday she was driving home and said the check engine light was on. I went to check it out and the light was not on. The light come on when the key is turned on but goes out when it is started. This is not an OBD-2 car it has a little blue plug and i do not have a code reader for it. Took it to auto zone and they do not have a code reader for this car. Auto Zone and Pep Boys both told me that if the light is off there will not be a code. Further checking on the internet I found out you can get a two digit code by turning the key on and off and count the flashing check engine light. It worked I got code 12 and 51. 51 means lean O2 sensor. If this is true that why does the CEL not stay on. How can I test the O2 sensors?
The Check Engine light will come on only if there’s a problem with the O2 sensor while it’s occuring under that driving condition. If you were to take the vehicle for a drive the Check Engine would probably come on again under that condition.
One way to test an O2 sensor is remove it from the vehicle. Then with a volt/ohm meter set at 2 volts DC, connect the red lead to the signal terminal on the O2 sensor plug and the black lead to the sensor body. The reading on the meter should be 0-.1 volts DC. Take a propane torch and apply the flame to the tip of the O2 sensor to remove the oxygen from the sensor. The voltage should slowly and steadily climb to .9-1 volt DC. Now remove the flame and the voltage should slowly and steadily drop back down to 0-.1 volt DC. If the voltage acts erratic while climbing or dropping the O2 sensor is defective.
On a 1994, the odds are the sensors need replacing anyway if it’s never been done. There are certainly shops that still have OBD1 diagnostic equipment, but you might have to call around a little to find one. The sensors for this car shouldn’t be horribly expensive if you get aftermarket parts.