Oxygen sensors - Code 41

Hi-Po

Member
Mar 31, 2005
106
0
16
Oslo, Norway
How can I test/check my oxygen sensors (HEGO)?

I replaced them maybe three-four years/approx 20/25,000 miles ago - and now I am getting Code 41 - "No HEGO (H02S*) switch detected - always lean (right H02S)


Just installed new heads/roller rockers/lifters/pushrods (Trick Flow), throttle body, EGR plate, MAF (Summit, 30 lbs), injectors (30 lbs), new fuel pump, new adjustable FPR, new IAC, new EGR valve and sensor.
Air pump and all smog systems are hooked up as per the stock design. Or at least I think so...

The CEL came on after the car had been idling for about 25 minutes (drying out some coolant in the exhaust).
Why did it take so long for the CEL to light up if the O2 sensors are bad?

I have set the fuel pressure at approximately 39 psi, though it may be a bit on the low side.
 
  • Sponsors(?)


jrichker

StangNet's favorite TOOL
SN Certified Technician
Mar 10, 2000
27,438
2,870
234
74
Dublin GA
lowendmac.com
If you use the wrong type of sealant on the intake or oil pan, the fumes from the curing sealant can damage the O2 sensors.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

Code 41 is a RH side sensor,
Code 91 is the LH side sensor.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC and the hoses connected to it.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides so that there is no carbon blocking the free flow of air to both heads.
 

93 LX

Founding Member
Jun 2, 2000
3,081
2
0
Norwalk, CA.
Visit site
When I wne to smog I used 70% rubbing alcohol which has allot of water in it. I passed smog but shortly after that i got the code 41. It would go out when I used the AC so I decided to replace them both, over 3 years old, and the code went away and MPG went way up.
 
Thread starter Similar threads Forum Replies Date
Luis911 Electrical I Had Code 92, Exchanged Oxygen Sensors, Now Code 42 (update Now Code 44 And 94) Fox 5.0 Mustang Tech 7
D Oxygen sensor 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 7
D **HELP NEEDED** O2 PIGTAIL WIRES 1996 - 2004 SN95 Mustang -General/Talk- 2
D O2 sensor harness 1994 - 1995 Specific Tech 0
D Oxygen sensor help 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 6
Onesick99GT Exhaust Wideband Oxygen Sensors Help Needed SN95 4.6L Mustang Tech 8
K Help P1151 SN95 V6 Mustang Tech 1
fredfifty Exhaust Leak At O2 Bung Fox 5.0 Mustang Tech 33
A Plesae Help With Oxygen Sensor Sorting Out Fox 5.0 Mustang Tech 6
D Emissions Problem - Very High Hc Fox 5.0 Mustang Tech 35
T Please Help O2 Sensor Hookup 5.0 Swap 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 7
T Engine 2005 V6 Oxygen Sensor / Cel Problem 2005 - 2009 Specific Tech 0
Blown88GT For Sale 2 Bosch Oxygen Sensors, Nib Engine and Power Adder 3
des89stang SOLD 87-93 Denso Oxygen Sensors, Nib. Other Classifieds 4
Blown88GT SOLD 2 Bosch Oxygen Sensors, Nib Engine and Power Adder 11
L Changing Front Oxygen Sensors SN95 4.6L Mustang Tech 5
L Oxygen Sensor Clean Up SN95 4.6L Mustang Tech 0
Luis911 Engine Useful Life Of The Oxygen Sensor Fox 5.0 Mustang Tech 6
M Removing Oxygen Sensor Questions 1994 - 1995 Specific Tech 3
J 95 3.6L V6- Oxygen Sensor SN95 V6 Mustang Tech 3
8 oxygen sensor socket SN95 4.6L Mustang Tech 5
93lxtwin Oxygen sensors Fox 5.0 Mustang Tech 9
90silverstang Running Rich,A9M ECU and Oxygen Sensor question Fox 5.0 Mustang Tech 13
arnie93lx 87-93 M/T oxygen sensor harness Engine and Power Adder 1
R WILL 2001 gt oxygen sensors work in my 96 GT SN95 4.6L Mustang Tech 1
Tripoli Upstream Oxygen Sensors - On with open loop tune? SN95 4.6L Mustang Tech 8
I upstream vs downstream oxygen sensor 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 2
hollywoodstang oxygen sensors and off-road H pipe 1994 - 1995 Specific Tech 16
J oxygen sensors SN95 4.6L Mustang Tech 5
M 99 cobra oxygen sensor harness SVT Tech Forum 2
FoxRod87 oxygen sensors SN95 4.6L Mustang Tech 5
babybudistmonk Oxygen Sensors 2.3L (N/A & Turbo) Tech 3
M oxygen sensor wiring-92 2.3L (N/A & Turbo) Tech 2
tx65coupe 1997 Cobra Oxygen Sensors SVT Tech Forum 2
C Tell Me If I'm Crazy. Oxygen Sensors SN95 4.6L Mustang Tech 17
V oxygen sensors question on a 97 4.6L V8 SOHC -which one goes where? SN95 4.6L Mustang Tech 0
V oxygen sensors on a 97 4.6L V8 SOHC -which one goes where? 1996 - 2004 SN95 Mustang -General/Talk- 2
blackstangt 4th wire on oxygen sensor harness. Fox 5.0 Mustang Tech 2
G oxygen sensor??? Fox 5.0 Mustang Tech 6
Blk91stang Checking Power to Oxygen Sensors?? How?? Fox 5.0 Mustang Tech 2
N Oxygen sensor question Fox 5.0 Mustang Tech 10
Quick Pony Oxygen Sensor Removal?? Fox 5.0 Mustang Tech 10
Chowder Head Does anyone have experience with this oxygen sensor kit? Fox 5.0 Mustang Tech 3
Quick Pony Oxygen Sensors ? Fox 5.0 Mustang Tech 3
chr1s11575stang Oxygen Sensors??? Fox 5.0 Mustang Tech 9
I Installed BBK longtube headers and engine runs better without oxygen sensors!!! Fox 5.0 Mustang Tech 8
Dean85GT Oxygen sensor bung thread size? Fox 5.0 Mustang Tech 2
P Oxygen Sensor Wires Backwards? Classic Mustang Specific Tech 3
stevesLX oxygen sensor wiring Fox 5.0 Mustang Tech 6
F Oxygen Sensor Signal Wire Fox 5.0 Mustang Tech 5
Similar threads