How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Matt Your Own Question
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 22893
Experience:  BEng hons Mech engineering, in auto industry 22 years
Type Your Car Question Here...
Matt is online now

my c180k runs perfectly well (good fuel consumption, smooth

This answer was rated:

my c180k runs perfectly well (good fuel consumption, smooth idling and good acceleration) but the eng management light comes on frequently. What is the problem? Code P0171 System too Lean (Bank 1), everything seems OK!!

I'd start by checking for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

also check the wiring from the O2 sensor for any damage and if there's no leak and no damage then consider replacing the sensor

Customer: replied 3 years ago.

I shall try your solutions tomorrow. Thanks. Where does the ECU get the information from to generates the code - the O2 sensor?. If so what is the probability of it giving false info. My car has done only 70,000 miles.

Hello John

yes the too lean signal comes from the O2 sensor

hence my suggestions

and the sensor can fail due to vibration , misfires poor fuel etc
Customer: replied 3 years ago.

Hi Mat,

Could it be that the sensor is giving a false reading? I say this because there is no leak before the sensor which is easily seen and the wire to it looks fine. The exhaust pipe connection to the manifold is only a few inches above the sensor and can easily checked for leaks. Furthermore, the constant readout of consumption is normal (32 miles/gallon on short runs and over 40 longer runs). Very confused - plugs in good condition, idles smoothly with good acceleration and exhaust gas is clear. Also checked the area around the Mass Flow Unit for induction leaks and and air filter was changed a few months ago.

In view of what you say about excess air in the exhaust. What do you think about a faulty Exhaust Gas Recirculating Valve allowing air into the exhaust when it shouldn't? I say this because the light comes on at low revs, idling and low speed. It does not come on when travelling at speed.




Hello John

its quite possible the sensor is giving a false reading and yes a faulty EGR could also give issues so it'd be worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
Customer: replied 3 years ago.

Hi Mat,

Having read the function of the EGR it seems the gas flow is from exhaust to induction manifold above idling speed when working correctly. The opposite to what I imagined.

Have I got this right and is it worthwhile cleaning the EGR. Is it possible that fresh air can feed into the exhaust under low speed conditions?

Hello John

thats correct, the EGR should be closedat idle and only opens at part load low to mid rpm

if it was stuck open at idle then you can get fresh in the exhaust and exhaust in the inlet

so general nastyness all round

if the EGR is OK then I'd take a look at replacing the O2 sensor
Matt and other Car Specialists are ready to help you
Customer: replied 3 years ago.

Thanks Mat. I shall only get back to you if I have run out of ideas!!