Hey Guys,


I have just replaced all 4 O2 sensors on my sons 97 x300. It has thrown the code 1194 which I had to look up: "Oxygen Sensor Heater Circuit Resistance Fault". It sounds like I might have gotten a bum sensor. If I make that leap how do I figure out which of the four I need to replace?


All suggestions on evaluating the sensors will be happily accepted.


Greg

 

I believe that code is for a downstream sensor. I would switch the sensors from the downstream to the upstream and see if the code follows the sensors.

If you choose to do that, I would change one bank at a time so you know which bank it is if the code follows.

I believe the equivalent code for the upstream is P1188, code. It could possibly be P1189, or P1187. Don't quote me on that.

 

Chris,

Thanks for the suggestion. Having just install all of these sensors, I am not in a hurry to take them all out to swap them around. I was hoping there is a way to test the sensors resistance in-situ to determine which one is bad. The downstream sensors are difficult to say the least.

Greg

 

To start with, it is the downstream sensors heater circuit. You can certainly test the heaters for resistance. I don't know the exact value but it should be fairly low- maybe 100 ohms or less. The two downstream sensor heaters are paralleled on the same circuit so the ECU cannot distinguish which sensor it might be. The wire colors on the vehicle side of the sensor connectors are UK and WB for the heater (Blue with Pink Stripe and White with Green).

So, what codes lead you to change the sensors? I am wondering if the sensor wiring circuit was bad before the sensors were changed.?

 

Hi Greg,

I am attaching the '96/'97 DTC Summaries manual, according to which, the possible causes of P1194 are:

1. ECM to HO2S heater wiring open circuit both upstream sensors (or intermittent open circuit)

2. ECM to HO2S heater wiring short circuit to ground

3. HO2S heater circuits high resistance or open circuit

4. HO2S heaters failure

As Ross mentioned, the most likely suspects are problems with the wiring between the O2S sensor wiring harness connectors and the ECM, not the sensors themselves.

You can download the '97 Electrical Guide at the link below, which will help you trace the wiring to check for a broken wire, chafed insulation allowing a short to ground, corrosion on electrical connectors or ground points causing high resistance, etc.:

http://www.jagrepair.com/images/Auto.../jagxj1997.pdf


Please keep us informed!

Cheers,

Don

 

Well, I have some work ahead of me.


I changed the sensors because I was getting a code Cat inefficiency. I did some homework and the prevailing wisdom is on this car it this error points to a problem with the upstream and downstream sensors. Given the Cats take a long time to wear out, changing the sensors is a first step. The is a '97 but the mileage is only 75k.

 

I too have had cat inefficiency faults due to bad lambda sensors. b I have also got those from a dodgy MAF and an air leak. It seems to be yoit another quirk of the X-300 OBD checks to give that code on a whim and not diagnose a misfire worth a flip!

 

Greg,

Something Ross posted in another thread just jogged my memory. I have a friend with a '95 X300 and he was getting codes for O2 sensors and misfires that couldn't be resolved with new coils and sensors. He finally traced the problem to corrosion on the ECM electrical pins and connector. That would be another check to add to your list.

Cheers,

Don

 

When I first bought my X300 eleven years ago I got a code that told me I needed to replace the O2 sensors. I replaced the front two. Everything worked fine for a while. Then same problem again. Took it to an Indy shop with a meter that could view the voltage time history of both sensors. They would work fine for a short time, that is voltage cycling slowly, and then they would go to flat line. Called Landcover Jag in Houston where I bought them and they told me I had over tightened them when they were installed. They can only be tightened about one half turn after the base makes contact. They sent me new sensors and these sensors had instructions with them that said the same thing. We did that and they have worked fine ever since.

 

Well I am checking the sensors one by one. I still have the old sensors (pack rat). I will try replacing them one by one to see if they were over-tightened.

Thanks everyone for the suggestions.