How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask ase_master327 Your Own Question
ase_master327, Shop Owner
Category: Car
Satisfied Customers: 5288
Experience:  +15 years experience on imports and domestics. Bumper to bumper diagnosis & repair.
Type Your Car Question Here...
ase_master327 is online now
A new question is answered every 9 seconds

1992 chevy: 4.3 litre truck started sensor..ecm

Customer Question

hi..i have a 1992 chevy c1500 pickup with a 4.3 litre truck started cutting out on me and throwing a code...i read the code and it was a code 33......witch is a map sensor high voltage code...i replaced the sensor...but it still does it and i get the same code....i traced the wires all the way to the ecm...and i dont see any problems...what else could be the problem...what else should i look at?.....ty.
Submitted: 8 years ago.
Category: Car
Expert:  ase_master327 replied 8 years ago.

No problem...Here is the diagnostic for code 33 on your truck:




Circuit Description:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1-1.5 volts at closed throttle (idle) to 4-4.6 volts at Wide Open Throttle (WOT) (low vacuum).

If the MAP sensor fails, the ECM will substitute a fixed MAP value and use the Throttle Position Sensor (TPS) to control fuel delivery.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

  1. This step will determine if Code 33 is the result of a hard failure or an intermittent condition. A Code 33 will set under the following conditions:
    • MAP signal voltage is too high (low vacuum).
    • TPS less than 4%.
    • These conditions exist longer than 5 seconds.
    • Engine misfire or a low unstable idle may set Code 33.
  1. This step simulates conditions for a Code 34. If the ECM recognizes the change, the ECM and CKT 416 and CKT 432 are OK. If CKT 455/469 is open, there may also be a stored Code 23.

Diagnostic Aids:
With the ignition "ON" and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check accuracy of a "suspect" sensor. Readings should be the same +/- .4 volt.

A Code 33 will result if CKT 455/469 is open or if CKT 432 is shorted to voltage or to CKT 416.

If Code 33 is intermittent, see Intermittents. See: Symptom Related Diagnostic Procedures\Intermittent Condition

  • Check all connections.
  • Disconnect sensor from bracket and twist sensor connections. Output changes greater than .1 volt indicates a bad connector or connection. If OK, replace sensor.

NOTE: Make sure electrical connector remains securely fastened.

Customer: replied 8 years ago.
i did theses procedures..and the map sensor has been replaced...and i stil have the problem..the truck drives fine and then when i go to give it the gas it has no throttle response and it stalls and throws the code #33....but the truck does start back up..and doesnt do it again until the lite goes out..then it will do it again. is there anything else that could cause the truck to throw a map sensor code.
Expert:  ase_master327 replied 8 years ago.
Then either it is not REALLY a code 33 you are getting, or you missed a step in the process.
Customer: replied 8 years ago.
could it be the tps...that causes this code..and its a code posotive of other codes in the could be a false code caused by something else...i just replaced the map leaning towards the tps.....could this be a good place to look?
Expert:  ase_master327 replied 8 years ago.

They get refwerence power from the same wire..It is the TPS, Coolant temp sensor and MAP on this reference signal voltage..That is the only way they are connected together.

High voltage means low vacuum, if the computer thinks the throttle is open farther than it really is, then it could POSSIBLY cause this problem, but it would USUALLY set another code for the TPS. Possible, but not the most likely cause.

So..Here are the instructions for testing the TPS:

  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impedance voltmeter at terminals C and B.
  5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
  7. If the sensor voltage is not as specified, replace the sensor.

Click image to see an enlarged viewFig. 2: Using a DVOM, backprobe terminals A and B of the TPS sensor to check for proper reference voltage Click image to see an enlarged viewFig. 3: Using the DVOM, backprobe terminals C and B of the TPS sensor, open and close the throttle and make sure the voltage changes smoothly Click image to see an enlarged viewFig. 4: Throttle Position Sensor (TPS) wiring diagram

Customer: replied 8 years ago.
Reply to ase_master327's Post: ok....all that checked was cleared of codes.i took it for a ride...and it did it again...i got home and read the codes and there was only 1 in there...and it was a code #42-wich is the ignition control module...and thats in the distributer....does that sound more like what the problem could be?

JustAnswer in the News:

Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.

What Customers are Saying:

  • I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry. Stephanie P Elm City, NC
< Previous | Next >
  • I would (and have) recommend your site to others I was quite satisfied with the quality of the information received, the professional with whom I interacted, and the quick response time. Thanks, and be sure that I'll be back whenever I need a question answered in a hurry. Stephanie P Elm City, NC
  • used your service this weekend with "Trecers" help. thank you ,thank you, thank you. replaced an A/C fan motor. Local Auto Zone had part. $15.00 "tracer" fee and $40.00 for parts, I saved several hundreds of dollers at a shop. i will recommend you and use you in the future. David L. Richmond, TX
  • 9 dollars, 2 hours of my time, and I drove away. Your diagnosis was right on the mark. Thank you so much. Phil Marysville, CA
  • Lurch. Thank you very much. I had real doubts about this website but your promptness of response, quick followup and to the point answer with picture was incredible. Charles Walnut Creek, CA
  • As a single woman, I really appreciate an excellent and affordable opinion.
    Thank you Geordie, I will not hesitate to contact justanswer in the future!
    Sue Charleston, WV
  • Another great insight to what may be the problem. I will have my mechanic take a look at it tomorrow. Thanks again, do indeed know your stuff. Jim Castleberry, FL
  • Excellent reply, and also very quick. Really sounds like the Expert knows what he is talking about. I will be back to use your service when I need more help with my RV. Dutch USA

Meet The Experts:

  • Chris (aka-Moose)

    Chris (aka-Moose)


    Satisfied Customers:

    16 years of experience
< Last | Next >
  • Chris (aka-Moose)'s Avatar

    Chris (aka-Moose)


    Satisfied Customers:

    16 years of experience
  • Ted G.'s Avatar

    Ted G.

    ASE Certified Technician

    Satisfied Customers:

    20 years auto repair experience, ASE Master Tech, Mechanical Failure consultant, Expert Witness
  • George H.'s Avatar

    George H.

    ASE Certified Technician

    Satisfied Customers:

    ASE Master Tech 15+ yrs, AAS Automotive Technology, Factory trained Asian specialist
  • GM Tech (Cam)'s Avatar

    GM Tech (Cam)

    Auto Service Tech

    Satisfied Customers:

    GM Grand Master Technician 2007. 14 years experience.
  • Amedee's Avatar


    Former ASE Master Tech

    Satisfied Customers:

    ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
  • Jerry's Avatar


    Master Mechanic

    Satisfied Customers:

    ASE master, 30+ years. All makes and models. Trouble shooter, shop forman, service manager
  • Richard's Avatar


    ASE Certified Technician

    Satisfied Customers:

    12 years Ford Lincoln/Mercury Jaguar dealership as a technician and shop foreman reparing all makes