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  OBD2 - Lambda

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OBD2 - Fuel Pressure
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OBD2 - Lambda
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OBD2 - Poor Running TPS
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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 needed.

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 output voltage.

Sensor output ranges from approx 0.2 volts (lean) to 0.8 volts (rich).

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 is HO2S22.

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 codes-

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 doing it.

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 re-occurrence.

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.





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