(Another) Engine Management Warning light

[Stu Sheridan]

Hi all,

New owner, 2010 Passat 2.0tdi Highline, flipping light has come on.

Took it in on Saturday and it appears it's the Mass Airflow Sensor. Pictures of the readout are below, hope you can make them out :





The fault code would not clear unfortunately.

Any advice on cleaning the MAS or where to buy and what to pay for a new one?

Given the fault code and tag line, MIL on, will a new one definitely cure the issue?

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[DMitch16]

Your latest code is an EGR fault? The MAF faults are months old.

I'd hold off the MAF purchase just yet. The codes should be cleared and then see if any come back.

It could be that the EGR is not functioning correctly due to a faulty N18 valve.

New, you would be looking at £100 to £120 for the Bosch OEM MAF unit. The VW part number for the MAF on your car (CBAB engine) is 03G906461C which will be a Bosch unit. Cheaper versions exist but questionable quality and longevity. I always fit the Bosch.

Second hand working anywhere from £25 upwards.

Easy fit but you might need a large pair of scissor grips as the spring bands are thick and a bit awkward to manoeuvre off.

Not sure the MAF is the problem though.

[Stu Sheridan]

Thanks Mitch. I'll do some research about the EGR and N18 valve you mentioned.

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[DMitch16]

EGR and antishudder flap units might need a clean. They get caked up over time and are sometimes incorrectly referred to as throttle bodies but Tdi engines don't have a throttle body! D

[Stu Sheridan]

Think you might be on to something with that. I've been on the phone to the previous owner - he had a MAS fault which cleared after trying a new MAS, finding no change and then swapping back. The record is there in the fault codes in May.

From what I've read, there is clearly interaction between MAS and EGR signals - the EGR is detecting excessive flow compared to what the MAS tells it to expect (think that's right?).

There is plenty of stuff on the net about p0402 - excessive flow - intermittent. The vast majority says start by cleaning and checking the valve isn't stuck.

The last time I did anything to an EGR I threw it in the bin and fitted a blanking plate. Get the feeling this could be more tricky!

I'll have look tomorrow hopefully, at the weekend if not.

[martin1810]

All air flow is detected by MAF. EGR cannot detect air flow. If MAF value is too high, ecu decides EGR flow must be too low. Simples. MAF is fairly reliable so fit a new EGR.

[Stu Sheridan]

All air flow is detected by MAF. EGR cannot detect air flow. If MAF value is too high, ecu decides EGR flow must be too low. Simples. MAF is fairly reliable so fit a new EGR.

Thanks Martin. So excessive flow would indicate MAF value too low. Think I'm getting it!

Car has done 67000 miles so I suppose it could be gunked up. The car has done less than 300 miles since it was MOT'd in April - would standing for a while have any effect on either component?

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[DMitch16]

The gunk would dry out a little making it quite sticky. Give the EGR and anti shudder flap unit a good clean with carb cleaner and check the flap is moving correctly. Otherwise replace the EGR unit as Martin says - they are around a £100 or so but the bolts are a little fiddly as the EGR pipe is stiff. I loosen the far end of the EGR pipe for a little more movement at the front.

[Stu Sheridan]

Thanks, will be taking a look tomorrow. Definitely got a little misfire when cold and takes a little longer to start first thing in the morning than it should, not really bad, 5 or 6 seconds of cranking.

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[DMitch16]

My 2007 Sport 170 with DSG and 162K on the clock kicks in first time almost every time still with no extended cranking even in the cold! Does have a tiny splutter sometimes whilst she clears her throat usually on a cold morning and spits out a blue oil cloud on first start but I know she needs new valve stem and injector seals.