Well, the DAMN! check engine light came on again for the 6th month in a row, boy i just want to kick my car now everytime i fix something within a few weeks something eles dies, hell and sadder it all started with the transmission going out, anyways, i pulled the codes after getting to work this morning and got 143 ( Knock Sensor ) 231 ( Lambda Sensor ) 121 ( Air Mass ), so i am comming to you guys first for symptoms of what i should be replacing first as the two work together when one dies the other goes off as well till you fix the one that's out, need some help here, was cruising at 80mph when it kicked off noticed immediately a huge drop in power accross the board at any speed, car still starts amnd idles but of course its alittle off, i decided to pull the plug from the air mass idle flew up, put it back on no change , someone said i should be checking the lambda sensor for voltage, but i dont know how to use one of those testers even though i actually have one, nor do i know how to hook the thing up to it ( its a 3 wire on the 89 ). Any help or suggestions would be greatly appricated as i just want my car back.

Before you replace any parts, check for vacuum leaks, check fuel pressure and even compression. A vacuum leak can cause both the O2 sensor the mass air flow and even the knock sensor by causing a misfire. Uneven cylander compression can cause any one of these codes also. Usually when there's more than one code at once, it's something completely different. I've seen dirty throttle plates cause a air mass meter code. Fuel pump relay's often cause a whole host of codes all at once.

I think you may have a AMM failure here.

The air mass meter depends on the measurement of current flowing through heated wires to measure air flow. It is also known as the hot-wire sensor because of its heated wire design, hence the "H" in LH. In the unlikely event that a wire should break, the warm engine runs, though without fuel compensation, in a "Limp-Home" mode. For "Limp-Home" operation, injector pulse time is fixed. For any rpm above idle, the ECU is programmed to deliver fixed pulses, typically 7.5 milliseconds.

The air mass meter works by measuring the air mass, or weight, so it requires no correction for changes in density due to temperature or altitude. The hot wire system depends on the measurement of the cooling effect of the intake air moving across the heated wires. With a small movement of air past the heated wires, the cooling effect is small. With more air moving past the heated wires, the cooling effect is greater. LH control circuits use this effect to measure how much air passes the LH hot wire.

The LH hot wire is found within the air passage tube of the air mass meter and is made of a platinum filament. The hot wire is heated to a specific temperature differential above the incoming air when the ignition is turned on (the differential is measured in degrees Celsius).

* LH 2.2, aluminum: 212F (100C)
* LH 2.2 and LH 2.4, plastic: 248F (120C)
* LH 2.4.1 and 2.4.2, plastic: 311F (155C)

As soon as the air flows over the wire, the wire is cooled. The control circuits then apply more voltage to keep the wire at the original temperature differential (100 degrees C). For example, if the air is at freezing, 32F (0C), the wire will be heated to 212F (100C). On a hot day, if the air is at 86F (30C), the control circuits heats the wire to the same 212F (100C) difference to 266F (130C). This creates a voltage signal which the ECU monitors - the greater the air flow and wire cooling, the greater the signal. These signals are transmitted from the air mass meter
Grab an LH 2.2 AMM (84-89), from a wreck in a junyard. Check the part # if you grab on off a Saab or 240 or something, or borrw onE from anothr working 740.

pull the knock sensor and take a ohmeter to it. you should get 4.8mohms of resistance to it. if not, it's busted.
--
Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'

The same thing was happening with my mass air sensor on my 91 940SE, I cleaned the contact point a half dozen times before I ended up replacing the unit, a bit pricey...but since it was replaced it has not happend again. (and that was a year and a half ago)

Hope this gives you some feedback