Returning code
Q
Can you shed some light on what may be wrong with my daughter’s 2004 Rover Streetwise 1.4 with the MEMS3 engine management system and 69,000 miles? She has owned it since 2005, with 3000 miles when bought.
The car has been regularly serviced by me approximately every 6000 miles, with the head gasket changed at 40,000 miles and 2.5 thou machined off the head due to a dished bow around cylinder 3-4 and oil in the expansion tank. I fitted a steel shim and an MLS gasket, new head bolts and a stronger lower ladder rail, water pump belt tensioner and camshaft oil seals. It still has a tappet noise when running, for which I fitted a new set of cam followers that made very little difference.
The car runs very well, but occasionally it throws up the EML code of ‘P420 – cat below threshold’, which I am able to reset for a few days before it appears again. This fault first occurred some two years ago, with similar freezeframe data, but I eventually cleared it for the last 18 months by changing two cats – first, the upstream oxygen sensor and, second, the post-cat sensor with an aftermarket one. The fault has since returned, showing ‘P420/13 – cat system below threshold bank 1’ on a Foxwell NT644 reader. The freeze-frame data is from OBDII ELM 237 Gendan software as follows: ‘P420 Cal Load 24.71%, Temp 82Deg C, STFT 4.69, LTFT -2.34, Intake Manifold pressure 71KPA, @3255 RPM, 85KM/H’.
I can’t see how the intake pressure can be that high. Is this the key to the fault? When the engine is run without being driven, hence a lighter Cal load, I don’t get anywhere near that pressure: ie, 24/28/33 KPA.
As this fault was cured originally by fitting a new aftermarket post-cat Lambda sensor, could this have failed or be beginning to fail again? Nigel Jenkins
A
Looking at the readings, I can see that the fuel trim levels are good and that the manifold pressure is well within its tolerances. The manifold pressure in an unloaded situation will vary greatly to a loaded condition and 100kpa is not unknown.
The problem may be a failed oxygen sensor, but I would first carefully check the wiring on the downstream sensor for any damage or connection problems. The engine coolant temperature reading is slightly low and if the engine is up to full running temperature then the sensor may not be working properly. Other points to check are any leaks from the exhaust system or the inlet manifold, although, given the manifold pressure, this is less likely.
The waveform of the 02 sensors can be checked on live data using your NT644 or by using an oscilloscope. The front sensor should have a smooth fluctuating waveform, while the waveform of the output sensor should be steady. If the output waveform matches the input, this would indicate that the catalytic convertor is faulty. A static output from the rear sensor would indicate the fault is with the sensor.