OBD2 Fault Codes - Poor Lamda sensors
Sanjay, a 24V Cosworth owner, put up a couple of scans on the
List reporting several errors showing on the CODES page of the Vehicle Explorer.
He reports -
Scan1 is a small file and shows the DTC codes when I first
connected the OBD2. These are P1120, P0151, P1151 and P0174.
Scan2 logs the sensors and is taken after I had cleared the codes (donít know
whether this was wise). The scan shows O2S21 at zero, O2S22 at a steady dc,
O2S11 switching and O2S12 following O2S11
My analysis is that O2S21 sensor is faulty and that the Cat on bank 1 is on its
way out. Interesting I just had the car MOTíd and the emissions check passed
with flying colours, but the performance is poor.
Any comments would be welcome as I would hate to replace something thatís not
This is an excellent example of how one issue can generate
several different codes.
Lambda sensors produces a voltage signal that recognises the
amount of unburned oxygen in the exhaust. An oxygen sensor is essentially a
battery that generates its own voltage. When hot (at least 250 degrees c.), the
zirconium dioxide element in the sensor's tip produces a voltage that varies
according to the amount of oxygen in the exhaust compared to the ambient oxygen
level in the outside air. The greater the difference, the higher the sensor's
Sensor output ranges from approx 0.2 volts (lean) to 0.8 volts
The lambda sensor's output voltage should not remain constant,
however. It should flip-flop back and forth from rich to lean as the PCM
increases and then decreases the amount of petrol injected into the intake
manifold. Every time the voltage
reverses itself and goes from high to low or vice versa, it's called a "cross
count." A good O2 sensor on an injection system should fluctuate from rich to
lean about 1 per second. If the number of cross counts is lower than this, it
tells you the O2 sensor is getting sluggish and needs to be replaced.
Most lambda sensors will cycle from rich to lean in about 50 to
100 milliseconds, and from lean to rich in 75 to 150 milliseconds. This is
referred to as the "transition" time. If the O2 sensor is taking significantly
longer to reverse readings, this too is an indication that it is getting
sluggish and may need to be replaced.
Observing the sensor's waveform using the OBD2 software is a
good way to see whether or not it is slowing down with age. If the sensor
becomes sluggish, it can create hesitation problems during sudden acceleration.
The figure below is the graph I ran of the Scan 2 - the red
trace is HO2S11 , Khaki is HO2S21, blue is HO2S12 and grey
It's very clear that the Lambda (02) sensor HO2S21 (Bank 2 Sensor 1,
which is passenger side) has flatlined, dramatically different to the
performance of Sensor HO2S11 in red. Because of this it has generated the
P0151, sensor low voltage, Bank 2 Sensor 1 (At the time this new code was
generated on Scan 2 the sensor was returning 0.00V)
P1151 HO2S21 lack of 'switch', sensor Indicates Lean, (This is seen by
the long periods of flatlining on the trace on the low reading. A lack of switch
on the high side would generate the error P1152)
P0174 fuel trim too lean, Bank 2. (The Fuel Trim, trying
to compensate for the flatlining sensor has run out of options in its lookup
tables and stores this code. This can be seen on the fuel trim on the HO2S21
which goes right up to the max at one point, clearly not even this enough to
provide a good combustion, hence the error. I expect that the LTFT (Long Term
Fuel Trim) was also maxing at the point this code was generated, but this data
was not selected during this scan. With the sensor faulty, fueling is being
controlled with the LT and ST Fuel Trim function, so it's not surprising that
performance is poor.
Yes, the HO2S12 sensor (Downstream Post CAT, Bank 1) is showing switching
which may indicate that the drivers side catalyst is on its way out. However,
when the switch rate between the front and rear sensor gets too high then a DTC
is generated - and this hasn't happened yet. There is the choice of sourcing a
catalyst now and replacing it, or waiting for the error code to come up and then
The P1120 is something else. It is the Throttle Position Sensor (TPS) voltage
out of range. Now this may be caused by a momentary radio interference as
well as a faulty sensor. You may like to check this with another reading. Start
to log with the ignition on and the engine not running, and select to read the
TPS. While you're watching the data, slowly depress the accelerator pedal and
watch the reading. Experience shows that it should start at about 16% and rise
smoothly upwards as you depress the pedal. Hold the pedal at odd positions and
check that the reading stays constant at that position and try another. If you
get a smooth progression and no other errors I would leave it for now and
continue to monitor the CODES page in case the same error re-occurs. It was
probably interference and the DTC would have cleared itself if there was not a
If the progression shown in the data is not smooth, or variable with no foot
movement, then check the connection to the TPS for a clean good contact. If the
good signal from the TPS cannot be restored then you should replace it as well.
Thanks to Sanjay for the scan.