99 Cougar dtc's p0174 p0171 p0420 tested and replaced parts but no luck.Help!


coogrr99
03-13-2009, 06:25 PM
my cougar 2. 5l keeps throwing these codes and i cant get to the bottom of it. ill start off by stating the work i already performed::banghead:
p0171-174
replaced -maf sensor,intake to valve cover hose(dry rotted),intake plenum gasket(had a leak i found with carb cleaner test),vacuum lines and elbows,air filter(had k&n went back to oem before maf was replaced),
AIT sensor(was throwing code for that. Cleared now)
i did a smoke test and found no leaks
p0420
replaced all (3) o2 sensors within last 6 months

i was told that aftermarket(not universal) O2 sensors might have problems. Should i replace bank1 with genuine ford 02?
could DPFE sensor, cause the 171/174 codes?the car has 95k!
any info would be highly appreciated

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discnik
03-14-2009, 10:51 PM
The P0420, Catalyst Efficiency Below Threshold, can be caused byu the sensor detecting the exhaust being lean for an extended period of time.

Since you have both P0171 & P0174 codes, lean bank 1 & 2, this is the reason for the P0420 code.

As the lean condition effects both banks have you tested for fuel pressure at the rail schrader valve ? Should be 45 - 60 psi. Key-On, Engine-Off.

If it is below the specified psi you could have a plugged fuel filter or bad fuel pump.

Have you checked the PCV valve and the lines connecting it. The polymer used for these fittings does not do well in a oil enviroment and can cause vacuum leaks that are hard to detect.

Have you checked for vacuum leaks using a vacuum gage ? Vacuum should be 19 - 21 Hg. at idle.

Check for vacuum leaks around the EGR valve gasket and anything else that is common to both banks and could allow un-metered air into the system or to the O2 sensors e.g. exhaust manifold gasket leaks.

Hope this helps. Private Message me if needed.

beyondloadedSE
03-15-2009, 09:22 PM
The P0420, Catalyst Efficiency Below Threshold, can be caused byu the sensor detecting the exhaust being lean for an extended period of time.

Since you have both P0171 & P0174 codes, lean bank 1 & 2, this is the reason for the P0420 code.

As the lean condition effects both banks have you tested for fuel pressure at the rail schrader valve ? Should be 45 - 60 psi. Key-On, Engine-Off.

If it is below the specified psi you could have a plugged fuel filter or bad fuel pump.

Have you checked the PCV valve and the lines connecting it. The polymer used for these fittings does not do well in a oil enviroment and can cause vacuum leaks that are hard to detect.

Have you checked for vacuum leaks using a vacuum gage ? Vacuum should be 19 - 21 Hg. at idle.

Check for vacuum leaks around the EGR valve gasket and anything else that is common to both banks and could allow un-metered air into the system or to the O2 sensors e.g. exhaust manifold gasket leaks.

Hope this helps. Private Message me if needed.

Wow...so much wrong information here. :shakehead

A P0420 (and P0430) is not caused by a lean code (or a rich code). Those codes are set when the cat monitor senses that the cat is inefficient. The cat monitor (in the PCM) determines the efficiency from the difference in the oxygen sensor switching rate. Prior to the cat, the sensor switches rapidly from rich to lean with the exhaust pulses. After the cat, the sensor should have very little switching. A lazy upstream sensor can cause those codes.

The fuel pressure should actually be about 38-42 psi at idle and about 50-55 psi at redline.

The vacuum intake pressure at idle is really about 10-20 psi depending on how warm the engine is. 19-21 Hg is about .3 psi. Who measures in Hg anyways?

That being said, generally the gaskets that fail are the intake manifold gaskets, which sounds like youve already replaced. Did you replace both the upper and lower intake manifold gaskets or just the upper? Personally, I replace parts with Ford OEM whenever I can, so if you could replace the O2 sensors with Motorcraft, I would. However, I wouldn't think replacing them with aftermarket O2 sensors would cause the problem to persist.

What's your gas mileage like? If its still around the typical 26-28 mpg, I say just reset the PCM and clear the codes.

coogrr99
03-16-2009, 12:22 PM
i did the fuel pressure test:49psi (key on position) / 43psi (running/idle) / 54-55psi (accellerated) is this around the right range.when i first put the gague on i had 22psi in key on position.after the vehicle was started for the first time of the day the readings above stayed solid(re-tested system 4 times).

Airjer_
03-16-2009, 12:43 PM
How about a dirty MAF causing the lean codes?

beyondloadedSE
03-16-2009, 08:06 PM
How about a dirty MAF causing the lean codes?

Possible, but not likely. Plus, he replaced the MAF sensor and its still throwing codes.

Did you replace the lower intake manifold gaskets or just the upper?

Airjer_
03-17-2009, 12:06 AM
oops! I missed that part, sorry.

tomcat228
08-17-2011, 10:57 PM
I replaced the fuel filter, and the two lean mixture codes went away!!! Service Engine Soon.
My total cost was $9.99 plus tax.

One end of the filter connection was a problem which I solved. Ford designed the engine side connection to require a special Ford tool for removal. I made my own. Worked great. The fuel line connector has 4 spring flat stock sections that need to be compressed allowing the flared end of the filter tubing to be pulled out free.

I used a small Rigid tubing cutter(1" diameter max.) to cut thru the tubing still connected. This allowed me to make a tool that would slide over the remaining piece of tubing for removal.

I made a piece out of Delrin on my lathe which worked perfectly.

After the simple reassembly, the engine started just fine, and the "Service Engine Soon" light was gone.
Now, I can have the car retested at the Illinois Air Team center, so I can buy my plate sticker for another year at a cost of $98.00. Good old Democrats raising fees and taxes.

If someone wants a tool like the one I made, I can make more for a reasonable price.




Had my 1997 Cougar with a 4.6 engine in for emission testing, and received trouble codes P0171 & P0174. Both sides of the engine are receiving a lean mixture of gas. How about looking at a 1966 Ford Mustang with a bad fuel filter. Lean mixture going to both manifolds resulting in poor acceleration if good at all. Does anyone remember how obvious it was to check/replace the filter?

I will do the replacement tomorrow, and will post the results to my error codes.

I'm reading way too many blogs about replacing expensive sensors that didn't solve the problem. Try the filter first!

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