Lukebolger

Hello!

Firstly, I am sorry if I have posted this in the wrong section, I was unsure which one to use!

I have a 2005 mercedes c160 coupe.

It has 66,000 miles on and is in very nice condition.

I have owned it about 6 months, and about 2 months ago it started using more fuel, and the check engine light came on.

I used my cheap ebay scanner to pull the code, it was

P0171 MERCEDES-BENZ - System Too Lean Bank 1

Anyway, I have been working lots but finally managed to take it in to the garage and have it looked at a couple of weeks ago

Now I was under the impression this was to do with the MAF sensor needing to be replaced.

The mechanic I'm dealing with, firstly said he couldn't find a problem with the car as I had re set the code.

So I had to book it in again, (I have left the check engine light up and not reset the code this time)

He thinks it needs new lambada sensors all round.

From what I could gather he didn't actually manage to pull the code with his diagnostics tool and accidently reset it.

Before I book it in to have my lambada sensors replaced, can anyone shed any light on if we think he's on the right track?

Thanks!

Luke

Fantomas

The code is triggered by your primary oxygen sensor because too much oxygen is detected in the exhaust. You have unmetered air in your intake. Could be your MAF sensor, could be a vacuum leak after the MAF sensor.

Find another mechanic, replacing oxygen sensors because they triggered the code is idiotic. Any professional mechanic should have access to Mitchells or Alldata, both of which have a step by step procedure for troubleshooting the code. The first step isn't to throw parts at it. Only the primary oxygen sensor meters the exhaust and provides feedback to the ECM, the secondary is only to monitor catalytic converter efficiency. The proper first step would be to smoke test the intake or use a can of starter fluid sprayed around the MAF and intake gaskets and see if your idle RPM goes up.

Lukebolger

Thanks for your reply.

The mechanic said, he's not getting any readings at all from the lambada sensor, hence it needing replacing.

I would have through if that was the case it would throw a code, specially for that, like my motorbike does when you disconnect lambada. Open circuit on o2 sensor or some such thing!

Fantomas

The fact that the car is throwing a P0171 code shows that it is getting a reading from the sensor. It would throw a completely different code if the oxygen sensor isn't working. Oxygen sensor gets a 5v signal on one wire from the ECU, which is then altered through the sensor depending on the volume of oxygen in the exhaust and sent back to the ECU on a different wire. The oxygen sensor need to be fully warmed up to read properly, that's why they come with a built in heater. If the oxygen sensor is broken it would report 0v or 5v back to the ECU triggering a short or an open code. Analyzing the freeze frame data is key, your fuel trims will describe the severity of the lean condition. The throttle position, RPM and temperatures may be useful as well.

Too much air is getting into the intake after the MAF due to a vacuum leak, the MAF is reading incorrect air volume, not enough fuel is being sprayed.