2001 mercury sable LS headache

2001 Mercury Sable LS - DOHC
188000+ MILES
Check engine codes: P0136, P0156, P0401

I just had the starter/flywheel replaced as well as a few other things. The check engine light came on almost immediately after I got the car back… And now this issue.

So, here’s what is happening:

As of late, the car has pretty much been very moody. It would drive pretty much perfect for hours, and then suddenly it would get pissy with me. It is coming to a complete stop (or damn close) on its own when my foot isn’t on the gas or brake. I even tested this going in reverse and downhill. It’s as if it is trying to stop itself. It should coast smoothly, but that’s not the case.

It’s being very, very sluggish when accelerating. Then yesterday and today, the car was shaking rather violently as I was driving (around 50mph). And it smells like something started burning.

I’m hoping my transmission isn’t about to completely fail. Do any of these error codes sound like the root of the issue?


P0136 = rear heated oxygen sensor signal, no activity, bank 1

P0156 = rear heated oxygen sensor signal, no activity, bank 2

P0401 = exhaust gas recirculation, low flow

I’m wondering if something wasn’t properly plugged in or connected, after the shop put everything back in place. Or maybe something was damaged during the repair . . .

I would bring it back to the shop and ask them to please recheck their work. Be nice, and explain that none of these problems were present before the repair

Am I correct in assuming that these problems all happened directly after the repair?


The check engine light definitely came up after the work was done. I’d love to get the car back there, as I’ve had it back for about 3 weeks. They knew about the issue the next day, but with my money being so tight, I couldn’t miss work. Herein lies the rest of the issue…

They are a very busy shop because they are usually very good mechanics. I cannot afford to leave the car there for very long as it effects my ability to work. When I brought it there for the starter/flywheel, I had to wait 12 days before finally getting the car back.

Do you think those errors are causing my issues? Does it seem like maybe a tune up is necessary?

The longer you wait could put more strain on whatever is wrong. Or the mechanic’s will blame it on something else.

I think they just forgot to plug something back in.
Call and ask if you could stop in an hour before they close and suggest that they just put it up on the lift and check the connections.

You may have to leave work an hour or two early, but not all day.
They should have at least one mechanic with an empty bay toward the end of the day.


@Yosemite they are always backed up at the shop. There is no chance of the car getting looked at before sitting a few days. I know I need to get it there, I was just hoping to get some opinions on what the problem is

It's as if it is trying to stop itself. It should coast smoothly, but that's not the case. It's being very, very sluggish when accelerating. Then yesterday and today, the car was shaking rather violently as I was driving (around 50mph). And it smells like something started burning.

After your next drive, feel the wheel hubs. A hot one means a dragging brake.


“Does it seem like maybe a tune up is necessary?”

It may very well be time for a tuneup, but those fault codes have nothing to do with a tuneup

I agree with @Yosemite, in that if they put it up on the lift, they might spot the problem(s) rather quickly

I’ll go off topic . . .

I’ve occasionally also tested/verified bad crankshaft position sensors in a similar fashion

Meaning, if it’s submerged in water that is still not even lukewarm, and the meter suddenly shows infinite resistance, the sensor is bad

more often that not, it’s the ones that don’t throw a code that get tested that way. But I usually do that on sidejobs in my garage, not at my daytime job

@db4690 I just recently had my car at a shop and it was up on a lift. I was having what seemed to be a tranny fluid issue. The mechanic who was looking at the car only found a minor leak and mentioned a few other small problems. He checked my plugs and says they looked fine.

To the best of my knowledge, I was given no reason to worry about my safety with any of the issues. What confuses me is the car driving perfectly fine for hours before starting to act up and fighting against me.

Maybe a dumb question, but is it possible that there is a problem with my tranny fluid disperal?Maybe the fluid isn’t able to properly do its job?


The shaking at 50mph . . .

If it occurred while cruising, the rims may need to be balanced

if it occurred while you were stepping on the brake pedal, your brake rotor(s) may be warped

That burning smell might be leaking engine oil or transmission fluid burning on the hot exhaust

I wouldn’t really want to make any guesses as to the transmission, at this time

Have you been getting the transmission fluid serviced regularly?

@db4690 Serviced, not quite. I’ve had to put fluid in several times though. I’m a car repair/maintenance novice. I am now beginning to finally learn what I can to maintain my car better. More specifically learning things I can do myself instead of having to bring the car to a mechanic for everything.

Just took the car for a short ride and had no issues accelerating and no shaking. Car wasn’t coming to a stop on its own. My brakes felt a bit weird, though. Like I had to apply more pressure than usual. Been driving my grandfather’s Camry recently and the brakes on that are very sensitive, so maybe that is throwing me off.

It’s so hard to pinpoint the issues as they occur sporadically.

*I’d like to note that I didn’t take the car on the highway on that ride. Topped out around 35mph. I’ll be on the highway in a little bit. Bringing it to my friend to take a look at it. He is a mechanic, so maybe he can come up with some theories.


In my opinion, you should have been changing the transmission fluid and filter at 30K intervals

That is something a non-mechanic could quickly learn to do

Your transmission is way overdue for a fluid and filter service. However, any possible damage will not be reversed by doing so.

If your friend does the trans service, have him drain the fluid into a tub. Then he can inspect what comes out

Since you will be doing some of the work yourself from now . . . on this vehicle and others, I assume . . . the first thing you need to buy is a decent jack and a set of 4 jack stands

Occasionally they have decent ones on sale at napa auto parts, for example. Some of the craftsman tools are also okay

Do NOT get one of those Harbor Fake aluminum “racing” jacks. You don’t need it

What you said about your brakes was somewhat disturbing. Do you have soft, spongy and/or sinking brake pedal?

Or is the brake pedal rock hard?

Soft, spongy and/or hard pedal = air in the system. Possibly due to a leak

Rock hard pedal = vacuum problem. Booster, for example. Or its vacuum source, etc.

I can understand a shop being very busy, but you complained about the CEL being on when you picked up the car.

They may be a great shop, but to just sending you on your way and not checking out the problem is not a very good business practice.
“We got our money” is the attitude I see here.

At that point they should have put the car on the lift, or at least read the codes and make another appointment if need be.

Time to walk with your feet and checkbook, they have not earned your business.

My question is, if the car was recently on the lift somewhere else didn’t you have them read the codes and and diagnose the problem.


You have added trans fluid before? How are you checking level? Engine on or off?

Alright, sorry I didn’t get back sooner. Was busy last night. I’m just going to post a quick update.

People knew about the codes, specifically the last guy that just had it lifted checking the trans fluid. I didn’t request he look into the codes as he wasn’t responsible for them.

Last night me and my buddy looked into the P0401 issue. He felt it was causing the other 2 error codes to trigger. He’s a mechanic also and was able to log into a Ford employee site. The problem was my car’s DPFE sensor. We went to AutoZone and picked up a new one. It was a bitch to find under hood. Took quite a bit of time with us both looking and we couldn’t find it. He went back into the Ford site to see if he can find a location. Eventually we found it on the website and on the car. It was a bitch to get to, but required no tools.

We replaced the old one since it was very broken and he reset the CEL so it was clear. Car was seemingly running much better, but I didn’t have a lot of time with it. Maybe 10-15 minutes driving. Pushed it up to 60 with no problems at all. Oddly enough, the brakes felt smoother as well. CEL hadn’t come on either, but I know I need more time.

I’ll be driving around tonight for a while, so I should a good idea if all the issues I was experiencing are resolved.

The picture is my old DPFE sensor.

DPFE == “Differential Pressure Feedback for EGR”.

This appears to be part of the EGR metering system. Cars before emissions requirements often had no EGR. Then with concerns about air quality in the 1960’s, EGR’s became necessary, and most if not all cars were equipped with them. The EGR function is to reduce combustion temperatures by feeding some of the exhaust gas back to the intake manifold, but only in certain operating situations (like during rapid acceleration) to reduce nitrogen containing pollutants out the tailpipe.

Over time the EGR function became more complicated. The engineers wanted to better control the EGR function, so it wasn’t just either on or off, but to feather it, a little EGR, but not too much, so to speak. It appears that’s what the DPFE is for, the feathering of the EGR function. My Corolla has some similar gadgets, operated both by vacuum ports on the throttle body, and electrically via the car’s computer, that do this same thing. I can’t for the life of me understand how they work, so I’m hoping they never stop working … lol …

If the EGR function is activated too much, or in the wrong operating condition, too much exhaust gas will be sent to the air intake, and the engine will stumble or stall. DPFE failure seems like it is specific to Ford products.


It’s a good plan to fix the EGR first, before working on the O2 sensor codes. They might never come back now the EGR is working correctly. Thanks for telling us the result, and best of luck.

Current update: been on the road a few hours. No real issues with accelerating or violent shakes so far. Pretty smooth sailing right now. That was the case the last few times I drove also, though. The acceleration and shaking randomly started after hours of being fine. Side note – CEL did come back on. I’m assuming it is the 0136/0156 codes again. Will have to get it checked out again.


Glad to hear the engine is operating smoothly now!

Was the new DPFE sensor metal or plastic?

Once you get your oxygen sensor situation sorted out, you really want to do a transmission fluid and filter service

@db4690 it is plastic. As was the old one. The car ran relatively smooth last night but I’m not sure I’m ready to claim my acceleration and shaking issues are solved. I’ll keep a close eye on it over the next few days.

I know I need an alignment. The car pulls to the right a bit and caused the front driver side tire to get worn on the inside. The brakes have been feeling crunchy on occasion (this goes back a while). Sometimes I can actually hear/feel the brakes crunching a little, other times not so much. I’m also getting a bit of squealing at times. Even when I’m not on the brake (this also goes back a while).