For the last month or so my check engine light has been coming on for a short while, then without apparent reason, going off. The OBD code has always been P0170. I recently cleaned the MAF sensor, which ended up canceling the P0170 code a few months ago.

Any idea what would cause the same code to periodically appear, then go away, then reappear again?

Any suggestions for diagnosis and /or repair would be greatly appreciated .

 

Hi,
This is classic symptoms for MAF near to range limit.
To confirm diagnosis you need to know LTFT (Long Term Fuel Trim). This can be read from the engine ECU with a good OBD scanner tool by anyone with an appropriate DIY tool.
With a system operating normally LTFT should be about 10 on both banks (ideally 0!).
I suspect that you will find it circa 25+.
After/during a particular journey the trim can exceed the threshold of about 30 at which point the next system startup checks create a P0170 and illuminate the CEL. However if the next journey takes the LTFT below this threshold the light will not illuminate because P0170 is no longer valid at startup.
A dirty MAF also tends to give a P0173 or similar (misfire) under heavy acceleration. P0173 behaves differently to a P0170 since it is not a startup check.
You have cleaned the MAF but the cleaning process only works with certain cleaners and if the deposits on the 'hot' film are the problem. The 'hot' film could be cracked in which case cleaning won't resolve.
What did you use to clean the MAF? My experience is that Toluene, not carb cleaner, works best unless Toluene is its main consistuent.
If the MAF doesn't respond to cleaning you need a new MAF (subject to LTFT value). These are relatively expensive. Cheap (non OE) ones presumably work but I would stick with Bosch.
You can get these occasionally on eBay NEW at about 80USD vs MB price of about 200USD. Take care with USED sensors since these may work but have a similar threshold problem to your own.
Stuart

 

Stuart,

Thanks for the details. I have an OBD tool and wil try to read the LTFT value.

I cleaned the MAF sensor early this spring using "contact cleaner". After cleaning the sensor the check engine light went off and stayed off for several months.

Since sending the original e-mail a few days ago the check engine light is off again.

I will probably clean the sensor 1 more time. It has 80K miles on it so I know it is getting old. When replaced I will go back with a Bosch unit.

Thanks again.

 

Just a follow up to the my earlier message.

The light came back on again so I removed the MAF sensor and cleaned it again with electronic contact cleaner. After 10 miles of driving the light went off and has remained off since. I've put appx 800 miles on the car since the MAF was cleaned and the car is running perfectly.

I assume the MAF sensor is just getting old and eventually will need to be replaced; it has appx 85K miles on it.

Thanks,

 

Hi,
I'm glad cleaning worked OK. Contact cleaner. I wonder what the active agent is?
Although MAF's get dirty I have no data on how long they will then run for. Mine continues to run fine and I have an expectation that it will do a further 75k or so.
The key is the Long Term Fuel trim. LTFT.
A value <15 is great. If it is up close to 30 you have problems.
If there was a significant shift after cleaning you have restored new life into the old unit.
If not you will soon get a reoccurrence of the problem.
Stuart