IPD sale XeMODeX - Experts in Volvo Electronics
Marking "Topic Solved" is functional again. It's the green check next to your post. BTW, please do use this when your question is answered and verified. Thanks!
MVS-Amazon link helps MVS!

Get email notification of topic replies. Log in or register (free). Amazon Link Buy anything with this and it helps MVS!

Volvo 850 Code 121 - Faulty MAF not faulty

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

Post Reply
Posts: 1
Joined: Thu Apr 26, 2018 2:09 pm
Year and Model: 1996 850
Location: Heinola, Päijät-Häme

Volvo 850 Code 121 - Faulty MAF not faulty

Post by steve92 »

I have a Volvo 850 1996 with an ECU LH-Jetronic 3.2 Engine: B5254S

A week ago when leaving to work the engine stumbled and shut off when leaving the parking lot like I was lifting the clutch too quickly but I didn't. Now it has been a whole week of investigating the issue the engine has.

Fault code: 121 MAF sensor value abcent or invalid.

Tested the MAF and it looked like it really was faulty due to the output signal voltage jumping all over the place. So I replaced the MAF with a new one but still the engine has this same issue and is still giving a code 121 after clearing it a bunch of times and even disconnected the battery for an hour.

So the next thing I did was testing if the MAF gets voltage, key in ON position without ignition, 12V input detected in MAF connector pins 2/3 (the sensor has a 4-pin connector). This seemed to be working as intended so next thing I did was testing the 3 cables going to the ECU that they are not shorted out or disconnected, and they were not. They were not shorted out to eachother either and the ground wire had a contact with the negative terminal on the battery as intended.

After a while of investigation I went to take apart the Main Relay (fuel injection) because according to a wiring diagram of Jetronic 3.2 the oxygen sensor, the MAF and the injectors were all connected to that main relay power source. I thought meaby that device was malfunctioning but it wasn't. The insides were as shiny as brand new device with no cracked soldering joints and even tested it with the engine running to see if it sparkles, and it didn't.

The ECU is in the safe mode but the engine wont stay running after startup. (stalling within 5-10 seconds)
So to keep the engine running I turned the throttle a little every few seconds to keep the engine running while investigating the Main Relay.
While giving that throttle a very delicate 3-5 degree turn I heard a sparkling sound coming somewhere from the middle of the fuel injectors.
I heard this sparkling on every turn of the throttle for 10-15 turns untill suddenly it didn't sparkle on the last turn and shot the RPM up to approximately 1100RPM for 2-3 seconds after which I didn't need to turn the throttle anymore to keep the engine running and it was somewhat normally sailing at 790-840rpm on idle. (tiny bit higher variable on rpm compared to a working system)

Fuel line pressure was tested and it had no problems. The whole fuel delivery chain should be fine.
Except having one noisy injector with a cold engine that Im already planning to check/replace or clean at some point.

However the only code is 121 and I don't know what to test here anymore. Can a failing ignition coil meaby cause a MAF Fault Code 121? As somehow I think it might have something to do with the sparkling I heard.

Any thoughts would be helpful.
User avatar
Posts: 5072
Joined: Mon May 25, 2015 2:30 pm
Year and Model: See Signature below.
Location: Kansas
Has thanked: 3 times
Been thanked: 647 times

Re: Volvo 850 Code 121 - Faulty MAF not faulty

Post by RickHaleParker »

Clean the MAF connector.

23: MFI 1-2-1 MAF sensor signal. Signal faulty.

DTC information MFI 1-2-1

S145881 Condition
Since the computed mass air flow is determined largely by the injection period, the system is dependent on the MAF sensor signal. The DTC is recorded if the control module detects that the signal is lower than 2.7 V or higher than 8 V.

Substitute value
The control module assumes a fixed load signal value (limp home) when the DTC is recorded. The adaptive and idling control functions are inhibited, and the ICM and AW 50-42 TCM load signal is adjusted to the full-load value.

Possible source
  • Open-circuit in signal or supply lead.
    Signal lead short-circuited to ground or supply.
    Contact resistance in connections.
    MAF sensor faulty.
Fault symptoms
1998 C70, B5234T3, 16T, AW50-42, Bosch Motronic 4.4, Special Edition package.
2003 S40, B4204T3, 14T twin scroll AW55-50/51SN, Siemens EMS 2000.
2004 S60R, B8444S TF80 AWD.
2005 XC90 T6 Executive, B6294T, 4T65 AWD, Bosch Motronic 7.0.
User avatar
Posts: 4577
Joined: Sun Dec 07, 2008 4:09 pm
Year and Model: '97 850 2.5 20v
Location: Serbia
Has thanked: 67 times
Been thanked: 176 times

Re: Volvo 850 Code 121 - Faulty MAF not faulty

Post by misha »

If your replacement MAF is NOT oem Bosch,it will not work.The fact is that the car will run better with an old one.
Disconnect the MAF and start the engine.If it runs better than you still have MAF problem.
Note that Maf sensors for Jetronic & later Motronic system are NOT compatible,neither the inserts.
They look pretty similiar and have the same number of pins but they are NOT interchangeable.
You need exactly the same Bosch MAF insert,with the same part numbers like old one.
Did you checked the codes from ignition ecu also?
'97 850 2.5 20v / fully equipped / Motronic 4.4 from the factory / upgraded with S,V,C,XC70 instrument cluster / polar white wagon
'91 Citroen XM 2.0 SI /fully equipped/mandarine red metallic
History of Volvos in the family:
'71 144 S
'73 144 De Luxe
'78 244 DL
'78 244 DL
'79 244 GLE
'85 340 GLS
Post Reply
  • Similar Topics
    Last post