Hello -- new user here. Girlfriend's '99 ML430 failed smog with P0150 and P0130 trouble codes. Know those are Bank 2 Sensor 1 and Bank 1 Sensor 1 respectively. Need help identifying which sensors and where they are so I can replace the right ones.

Any help is appreciated

 

Hello and Welcome -

O2 sensors are usually located in the exhaust system just aft of the exhaust manifold. If you have two per side, the front one is before the catalytic convertor and the second one is after. On models of that era the wires are usually blue and that can aid in finding them.

BTW, Bank 1 is the passenger side (left). It's kind of rare for both to fail at the same time but since they have a life of around 100K it's probably worthwhile to change both.

 

Thank you Richard. Any idea of the size of the wrench needed to remove them? Special tools? Looks pretty tight, especially on the driver side. Is sensor 1 Pre-cat?. Could it be a sign something else is going on upstream of the sensors? Girlfriend says check engine light has been on for some time so it's conceivable they've been bad for awhile, but then again dont want to spend the money on that if it's actually an issue with something else, i.e. intake leak, MAF, etc...

 

22mm wrench. They make special o2 sensor wrenches but a 22 should do the job just fine

 

The standard size for oxygen sensors is 7/8" or 22mm. I haven't had to change mine but I know there are a number of dedicated tools including a deep socket with a 'cutout' on the side for the wire made for removing them. Failing O2 sensors can be a problem as the fuel mixture into the cat can get too rich causing the catalyst framework to melt. They often make a rattling sound when you hit them with the back of your hand or even hit a bump in the road. So I wouldn't procrastinate since the cats are way more expensive than the O2 sensors.

If you don't have one, you might get an inexpensive OBD II scanner (Harbor Freight about $50) and use it to clear the CEL then rescan when/if the CEL comes back on. Sometimes codes trip other codes.

 

Thanks again Richard. I ordered thru Autohaus this morning. Always put off getting OBD scanner for some reason but all of the trips I've made to Autozone to use theirs over the years has gotten tiresome

 

Many people out here have purchased the Harbor Freight #94169 scanner for $59 but often on sale for $39. It has been discontinued. The new one #98568 appears to be the same scanner but multilingual. Normally, $59 but on sale at the website for $49. If you printout the online ad and take it to a HF store the manager will usually honor the sale price.

autohausaz is a good place to get a genuine BOSCH O2 sensor. Some cheaper eBay sensors are either a similar VW sensor or Chinese clones. Most people have had trouble with those.

 

A 22 mm crows-foot wrench is a good alternative to an open end, helping to solve the problem of restricted working room.