Where are the locations of the oxygen sensors on a 2005 Gran Marquis LS


Asked by Dec 11, 2013 at 05:53 PM about the 2005 Mercury Grand Marquis LS Ultimate

Question type: Maintenance & Repair

I understand there are two sensors. How much trouble for me,  a "shade tree mechanic" to replace them??

14 Answers

Tracy Hooks

Question: Why are you replacing them? They rarely fail together unless contaminated. There are two Oxygen sensors and two Catalyst Efficiency Sensors. The Oxygen sensors are in the exhaust pipe within a few inches of the exhaust manifold. Best seen from under the car - you may be able to access the left one through the fender well but no promises. The connectors are a little tricky since they lie just on top of and beside the Transmission bell housing. The Downstream sensors are in the exhaust pipes near the end of the transmission. 7/8" 02 socket and wrench. Be sure to tie up the wire harness to keep the wires off of the exhaust. Give us a clue as to why you're changing them and lets find out what's going on with your car.

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Apparently my 1st response to this never materialized so here goes #2. Tracy thanks a bunch for your knowledge. Much appreciated. I'm not changing anything right now Tracy. I like to get all options on the table before I jump with both feet. Done too much of that in the past and ended up becoming 3" shorter. The Merc is my wife's car with 50.6K miles. She takes really good care of it, so when the "Ck engine light" came on last weekend she went ballistic. The first thing I did was check the gas cap. It was tight but I retightened it again to 4 clicks. Nothing changed. I've read in this forum to put a touch of Vaseline around the outside edge of the cap and/or clean the outside/inside edge of the fill nozzle. WHAT DO YOU SUGGEST? I then read that it could be one or both of the O2 sensors, hence my ?'s for location, in case I want to try and replace them myself. Since I'm cheap and didn't want to spend $$ on getting a reading on the light and then having to pay the dealer for the replacement, I went ahead and bought an Autel AL519 scan/reader tool yesterday. Haven't used it yet but hoping it will be today. So with all of that Tracy, what else can you suggest??? Thanks Again, Phil,

Tracy Hooks

The Check Engine light will generate DTC's (Diagnostic Trouble Codes) that will lead you in the right direction. The scan tool will also let you reset the Check Engine light. Retrieve the codes and we'll see what you have going on. The Gas cap is the most common failure but there are other things that can trip it. (BTW - I work on a fleet of 800+ Crown Victoria Police Interceptors 1999 to 2011. The 2008 and 2005 models are our most common (not that there's a lot of difference between the two!)

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Tracy I finally got my 519 updated and within the hour I'm going out and hook it up. I'll let you know what happened.. Thanks Again. Phillie,


Tracy here goes.. I got a P0401 $07E Generic Exhaust Gas Recirculation Flow Insufficient Detected. On the IM Readiness DTC Guide Page it said.1... EERG does not have sufficient Vacumn to Operate. 2..Faulty Delta pressure Feedback of EGR (DPFE) sensor. 3. Blockage in exhaust gas recirculation (egr)tube 4. Faulty exhaust gas recirculation (egr) sensor Modules Present..ID ($07E8) and Protocol (CAN STDFAST). So with all this info Tracy what do you think is the problem and where do I find it. O Key Dokey,, Phillie,

Tracy Hooks

It's very likely that the DPFE sensor on the top of the EGR valve has failed. No surprise with all of the hot exhaust gasses it sees. If there's no damage to the wiring (accident, rodents, fire) and all of the vacuum hoses/tubes and wiring are connected - it's likely that you'll need to replace the ESM (Exhaust Gas Recirculation Module). Ford decided to integrate several sensors and actuators into one component. Not a bad idea for the money (separate sensors and actuators can get expensive) but then one component can look like several failures. The code you're getting points toward the DPFE sensor (in layman's terms - the sensor that measures exhaust back pressure and EGR flow). With the right scan tool, you can watch the readings for the sensor, actuator, supply voltage and ground. With a really good diagnostic scanner, you can actuate the EGR solenoid and watch the DPFE readings change (or not). OK, so - Best bet is to change out the EGR (ESM), clear the codes and take it for nice long drive. P.S. - it won't damage the car to drive it for a while with this failure but your gas mileage and performance may suffer a little.

1 out of 1 people think this is helpful.

Tracy thanks again but unfortunately the diagram is too blurred to understand. Any chance of sending it out again or if possible tell me where you got it and maybe I can go to the site and download it. Phillie,


Tracy in the meantime I located the EGR valve. I believe the DPFE sensor is on top of the valve and I removed it (hopes it right). I since been going to all the parts house sites but none of them come up with just a sensor. They have an ERG valve assy with what I took off included for $177.00 big ones. I would imagine the sensor would be a lot cheaper but don't know where I can get one. I'd like to go that route first but with my luck, it could end up not being the problem so then I would have to buy the ERG valve anyway. Whatever thoughts you have would be greatly appreciated. Thanks Again. Phillie,

Tracy Hooks

Sorry but you can't get it separately. The price is about right though. NAPA has good emissions-related parts - not cheaper but good quality.

1 out of 1 people think this is helpful.

O key dokey then. I'll bite the bullet Tracy and I can't thank you enough. But hey since I got the piece separated, I'll spritz some carb cleaner on it and put it back on and see what happens. I'll let you know....

Keely Williamson

Tracy I Need Some Help. My Car Exhaust Pipe Has A Hole And My Floorboard Under The Drivers Seat Is Getting Very Hot... I Can Feel The Heat Inside My Shoes. So My Temp Light Comes On And My Car Loses Power. Its Automatically Slows Down And Shutter Until It Shuts Off. Any Solutions?

Tracy Hooks

It's very likely that you have a plugged Catalytic Converter. You're really going to have to visit a muffler shop for this one.

Keely Williamson

Ok Thanks Will Do So.

Keely Williamson

Hey Tracy. Well they cut off my catalytic converters and car still shuts off after i hit the gas and temp light comes on before it dies completely. It also makes a popping noise under car when i hit the gas

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