I was in service and they removed the FAP filter and programmed the engine computer so that it would not ask it.

But there were still errors and they said I need to change the upstream sensor. I asked what bad can happen if I drive with not working sensor. They said the engine can overheat.

When I went to another service to replace the sensor, they said this is nonsense. They said that first service done programming work badly. And so they reprogrammed, and said this sensor is not needed.

I also googled and did not find info about overheating engine. I found something about exhaust system, which might make sense.

So did the first service lied/or do not know what they are talking about?

My car is peugeot 406, 2.2 HDI

2 Answers 2


I've never heard of a bad O2 sensor - upstream or downstream - causing overheating. If the computer can't read the sensor, it will tend to run rich - i.e., more fuel - which will actually produce cooler the exhaust and, by extension, remove a little more heat from the system.

Unless there is something really odd about your Peugeot, the first shop was incorrect.

A bad O2 sensor will, however, reduce fuel economy. If left uncorrected for a long time, it also can damage your catalytic converter(s).

  • Does it matter when my FAP filter is removed? I am not sure is FAP filter and catalytic converter in one unit in my car, and so I am not sure if catalytic converter is removed.
    – Darius.V
    Commented Aug 2, 2017 at 17:26
  • @Darius.V The FAP filter traps particulate matter to keep wonderful blue diesel exhaust from coming out of the back. I'm not sure why they would remove it, but I also don't see how that could cause overheating.
    – 3Dave
    Commented Aug 2, 2017 at 17:33
  • They remove it because it is much cheaper than to buy new one and replace. My car is made at 2002, so its not worth to invest ton of money into car which costs like 1000-1500 eur in the market.
    – Darius.V
    Commented Aug 2, 2017 at 17:36
  • @Darius.V that makes sense to me. However, removing it may prevent your car from passing emissions testing, if you live somewhere that is required.
    – 3Dave
    Commented Aug 2, 2017 at 17:38
  • I was thinking and trying to avoid removing it. But no matter what I have done - like driving on high revs, after 2 years it started not help. And there were 3 options - remove it, find used one and replace with used or buy new one. THere's risk with used one. But shops and lot of people say it still passes emisions testing if the engine works correctly. One shop said - half of diesel cars in my country are without those filters. I am from Lithuania. Guy even said that emmision started to be better without that filter, but he did not pass because it visually was visible that it is removed.
    – Darius.V
    Commented Aug 2, 2017 at 17:40

Yes, no, I don't know. The heated exhaust gas oxygen (HEGO) sensor is used to establish fuel air ratios on some vehicles. You could run rich or lean. I've personally seen a vehicle with long term HEGO fault that caused continued preignition to to point that the bottom of the pistons broke off past the rings. Vehicle overheat... no. The pistons broke. Hint: that's a mighty expensive repair.

Recommendation. Get vehicle to design intent. Repair or replace all inoperable sensors. They are pretty cheap to replace. If you are in the USA Autozone has an awesome tool in the free tool loaner program that makes this a ten minute repair for the Do It Yourself person. The biggest problem is the repair is best done on a hot exhaust. You need to be careful to not burn yourself.

special tool

I will say, I'm a bit confused by your posting. What, pray tell is a FAP filter? I have no clue what that is. Best practice is to define mystery abbreviations When First Used (WFU).

You also mentioned the dealer "programming" the engine computer. That sounds pretty odd to me. In my experience (not with Peugeot, but with a United States based Manufacturer) engine computer reprogramming is a mighty rare event. It does happen, but not very often.

Its more likely that the dealer reads stored fault codes (OnBoard Diagnostic (OBD2) codes, and erased them. Is that a possible interpretation?

I wouldn't necessarily call either service team dishonest... Best action for the long term reliability of your car is to fix defects to manufacturer's design intent.

  • FAP filter - this maybe will give you an idea: youtube.com/watch?v=oQfBCQek6eg I think they are really programming computer. When they erase error code, they tell they erase. You cannot get rid of errors forever if you do not change the code of computer I think after removing FAP filter. Even if you erase, it will show them again after some time. And the shop which reprogrammed 2nd time, said its expensive to replace that sensor, new one costs like 180 eur they said. My car might cost in market 1000-1500 eur. To buy new FAP filter it cost I heard about 700 eur.
    – Darius.V
    Commented Aug 2, 2017 at 17:33
  • Btw you talk about preignition. Are you maybe talking about petrol cars? My engine is diesel. And diesel gets burning from the pressure, not ignition if I pick english words correctly.
    – Darius.V
    Commented Aug 2, 2017 at 17:35
  • Ouch. Yes I was talking about gasoline engine. How did you tell me that? I've never worked on diesels and emissions. So I'm still confused. What does the abbreviation FAP stand for? I believe you are talking about Diesel particulate filter (French, filtre à particules). Again possible to get you to communicate better with others on the site. Am I the only person who is highly offended at the use of unexplained mystery abbreviations?
    – zipzit
    Commented Aug 2, 2017 at 22:29
  • FAP stands for as you mentioned. I see in this forum peugeotrczforum.co.uk/what-does-fap-stand-for-t3033.html - "Filtre a Particule". Maybe its mystery for you when you have not worked with diesel engines. As I understand David Lively did not have a problem about that, so maybe it is not a mystery in general, but its a mystery when you don't know. I do not see there a problem. Maybe someone from Africa does not know what is peugeot, should I also tell this in original post? When you asked, I answered what is FAP - no problem.
    – Darius.V
    Commented Aug 3, 2017 at 17:03

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