Re­turn­ing code

Car Mechanics (UK) - - Diagnostics Doctor -


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


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.

The mod­i­fied gas­kets and lad­der rack as fit­ted by Nigel Jenk­ins.

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