Tip & How-To about Mercedes-Benz E-Class

How to check and fix oxygen sensors

1.12b) How to check and fix Oxygen sensors



How to check? An issue with oxygen sensors is that they steadily lose effectiveness with age and they can under perform for quite a while before they trigger an error code on the ECU. Ensure the connection to the oxygen sensor is robust and clean. As the oxygen sensor only works when hot there is the danger of getting burnt by working on it so a safe approach is to find the electrical connection on the wiring harness remote from the oxygen sensor and to make voltage measurements there. If you have multiple sensors and one sensor is reported by DTC as having failed it can be confirmed by clearing the error code and swapping sensor connections over. If the error code reappears but is now on the 'new' sensor then the previous sensor is at fault. The best approach is to remove the sensor and inspect it. If the sensor tip is black with soot this means that the engine is running very rich. If white this means there is antifreeze or silicone contamination. The sensor should be grey. If the sensor has four wires inspect carefully where they attach to the sensor. Generally if two wires are of the same colour they will be responsible for supplying the heater element with power. Check the resistance between these two heater element wires, it should be low, between 2 and 16 ohms depending on manufacturer. If a short is present this will also show up as low resistance and this will disguise the fact that the heater element no longer functions in this instance heater appears to be good but when operating it soon becomes faulty.

The other two wires are the signal output and earth; the earth wire (often grey), where it exits the sensor, is often offset close to the metal body of the sensor. Put a voltmeter across the sensor wires, red voltmeter lead to the signal wire (often the black wire) and black voltmeter lead to the grey 'ground' wire. Hold the sensor tip in the flame of a butane burner. The supply of heat and absence of oxygen should cause the output voltage of the sensor to climb to 900mV or more after about 15 seconds.Removing the sensor from the flame should cause the voltage to drop to less than 100mV within a few seconds. If the sensor is caked in carbon deposits the minimum voltage may be kept high (300mV or more). It might be possible to 'repair' a sooty sensor by heating the tip within the butane burner flame until red hot to burn off the deposits. Once repaired, the sensor will again have a much lower minimum voltage (<100mV).

The oxygen sensor can be tested away from the car using a rag dampened with WD40. Attach a 12V supply to the heater wires and measure the voltage across the sensor wires. Wrapping the sensor in the WD40 dampened rag should immediately show voltage output. If the voltage begins to decline whilst still wrapped up it is likely that the heater element has a thermally induced break in it and the sensor should be replaced.

To test the sensor in situ in the exhaust put a voltmeter across the sensor wires. Switch the engine on. When the engine is cold the engine is in open loop mode and the ECU will ensure that the fuel air mix is very rich and as a consequence as the heater element warms the sensor voltage should rise to >0.9volts. This voltage is maintained until the engine has warmed up and the coolant sensor has signaled the ECU to enter 'closed loop' mode. At this point the voltage from the sensor will drop to about 0.1volts as the ECU reduces the injection cycle making the air mixture leaner. When the engine is cold the sensor should show, within 20 seconds, an increasing voltage from the sensor wires. If not it is likely that the internal heater element has burnt out in which case a new sensor is needed. This will be confirmed when the exhaust eventually becomes hot, supplanting the role of the heater element, and the sensor reports an increased voltage before entering closed loop. If there is no response at all from the sensor at any part of the engine cycle then the sensor will need to be replaced


How to fix? If the burning off of the residue trick does not work, then replace the sensor.


Additional note: Auxiliary/Secondary Air Injection

On some cars (most notably VW and Audi) at cold engine start an electrical secondary air pump with a valve, pumps additional fresh air into the exhaust manifold. This input causes the Oxygen sensors to sense and then send a 'lean' condition (low voltage) signal to the ECU. The ECU responds by significantly increasing the injection times. Not all the added fuel is burnt and is carried through in the exhaust where it allows the catalytic convertors to reach operational temperature much more quickly and thereby reduce harmful engine emissions.

On some cars (most notably GM) the secondary air injection into the exhaust manifold is governed by a solenoid switch valve that opens and closes vacuum from the inlet manifold which in turn opens and closes a larger actuator valve. The complexity of this system creates a large number of points of possibly failure making quick diagnosis difficult and the repair costs high.



NEXT 1.13) FPR - Fuel Pressure Regulator


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CHECK LIGHT ON WITH A P0420


Error Code: P0420
Fault Location:
Catalytic Converter System, Bank 1 - Efficiency Below Threshold
Possible Cause:
Leakage in Intake and/or Exhaust System.
Catalyst faulty.
Oxygen Sensor(s) faulty.
Oxygen Sensor(s) Control faulty.
Possible Solutions:
Check Intake and Exhaust System for Leaks.
Check Catalyst.
Check Oxygen Sensor(s).
Check Oxygen Sensor(s) Control.
Perform Oxygen Sensor(s) Aging Check.

Mar 19, 2013 | 1999 Honda Civic

1 Answer

2001 escape 6cyl revving in park and neutral. Recently replaced TPS to fix a low idle problem as well as new intake gaskets upper and lower to ASSURE no vacuum leaks?... any help appreciated


The TPS would have been my first suggestion, but, something that can cause similar idle issues is your Oxygen sensor, ( although that generally triggers your check engine light. )

A bad oxygen sensor inables the computer system to determine the proper fuel / air ratio.

You may also have a sensor on the transmission itself that effects the idle, I know some vehicles do, I'm just at a lose for the moment, of the correct name of the part.

I hope this helps you.

Jan 29, 2011 | Ford Escape Cars & Trucks

1 Answer

I am wondering if anyone else is having fuel injection problems? My 98 S-10 ext cab with 4.3 Vortec with 96000 miles is drinking gas (only 12mpg) and has a strong smell of gas from the tail pipe. I am getting info from area mechanics that this is a common problem with the spider like injection system. I have changes plugs, wires, fuel filter, cap, rotor, egr, cleaned throttle body, and cleaned mass air flow sensor. Anybody? Help!!!


I can add two two extra possible causes: 1) faulty coolant temperature sensor/low coolant/air in coolant. When the engine is started from the cold the ECU enriches the fuel mix to keep the engine idling; this explains why engines idle high (1000 -1200rpm) at start up. As the engine warms up the coolant temperature sensor signals this and the ECU shortens the injection cycle to eventually bring the engine down to a steady idle (700-800rpm). How to check? Most often the coolant sensor is quite separate to the temperature sender, so a correct read-out on the dash board does not necessarily indicate correct sensor function. Using a voltmeter the resistance across the electrical terminals on the sensor can be measured. By removing the device from the car and putting the end of the sensor in a pan of hot water it should be possible to see an immediate change in resistance, it does not matter so much that the resistance goes up or down but that there is a disernable resistance change with change in temperature. Generally high resistance equates to cold temperatures and vice versa. If there is no resistance change commensurate with temperature change then the sensor is at fault. If there is simply no resistance measurable (open circuit) then the sensor is at fault. If the sensor is working correctly check the connector, the wiring and the wiring insulation for faults and possible shorting.


2) faulty oxygen sensor on the exhaust manifold. If the oxygen sensor indicates that there is too much oxygen in the exhaust the ECU will enrich the fuel mix to compensate. How to check? An issue with oxygen sensors is that they steadily lose effectiveness with age and they can under perform for quite a while before they trigger an error code on the ECU. Ensure the connection to the oxygen sensor is robust and clean. As the oxygen sensor only works when hot there is the danger of getting burnt by working on it so a safe approach is to find the electrical connection on the wiring harness remote from the oxygen sensor and to make voltage measurements there. Most garages have systems that can record the amplitude and frequency of the voltage peaks being produced by the oxygen sensor. A less sophisticated means to get some impression of the oxygen sensor function is to use a moving coil galvanometer type voltmeter (analogue needle on dial). Setting the voltage range to 1 volt and by attaching the meter leads across the sensor wires it should be possible to see the rhythmic pulsing and the voltage range of the operating sensor output. If no pulses are seen it could be either a break in the wire or a fault with the sensor itself.

Dec 09, 2010 | 1998 Chevrolet S-10 Pickup

3 Answers

my 2000 pontiac montana wont start,it just turns over,both oxygen sensors are shot and the egr valve is stuck closed,which one would be the main problem


The EGR will not open at idle or when cold, so if it is stuck closed this will not affect anything. If the O2 sensors are reporting 'too rich' (not enough oxygen) signals, the ECU will respond by leaning to fuel injection cycle to the point that there is not enough fuel to start.

How to check? An issue with oxygen sensors is that they steadily lose effectiveness with age and they can under perform for quite a while before they trigger an error code on the ECU. Ensure the connection to the oxygen sensor is robust and clean. As the oxygen sensor only works when hot (they have their own in-built heaters, hence two wires for heating and two wires for sensor operation) and so there is the danger of getting burnt by working on them so a safe approach is to find the electrical connection on the wiring harness remote from the oxygen sensor and to make voltage measurements there. Most garages have systems that can record the amplitude and frequency of the voltage peaks being produced by the oxygen sensor. A less sophisticated means to get some impression of the oxygen sensor function is to use a moving coil galvanometer type voltmeter (analogue needle on dial). Setting the voltage range to 1 volt and by attaching the meter leads across the sensor wires it should be possible to see the rhythmic pulsing and the voltage range of the operating sensor output. If no pulses are seen it could be either a break in the wire or a fault with the sensor itself.

Your vehicle is about 10 years old and most oxygen sensors last about 10 years or 100,000 miles....check to see how easy they are to get at and what the price is for replacement before contemplating replacing them

Nov 11, 2010 | 2000 Pontiac Montana

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