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Two things commonly cause the P1100 code to be mistakenly thrown.

  1. Most often, it's because the engine control unit (ECU) has become faulty through age or, rarely, because of bad internal software. In the latter case, there is very likely a technical service bulletin (TSB) on the problem.
  2. Another common cause may be because of a bad MAF or MAP sensor and an incorrect reading of the problem by the ECU.

When a P1100 occurs on a vehicle with no barometric sensor, the first thing to do is to find out if there are any recalls or TSBs issued for the vehicle in question. This information can usually be obtained from your local Toyota dealer by asking them to run your vehicle's identification number (VIN). Many dealership shops will do this for free. Some independent shops will as well.

If there are no bulletins for your car's MAP, MAF, or ECU, then you'll need to test the MAF/MAP for problems. Most mechanics will recommend removing and checking them for buildup, but in the case of a P1100, this may not clear the problem as it's likely in the sensor's electronics. Some sensors can be tested, but the cost and effort required is likely about the same as replacement costs, so weigh your options first.

With the ECU, testing is a must as replacement costs are likely going to be high. Some well-equipped repair shops can do these tests, but it's likely something that will be referred to the dealer's specialized shop instead.

Before going to all of this trouble, however, you may have a simpler solution.

Find out what the actual problem was that triggered the code in the first place – likely one of the other codes included in the array that tripped will be the actual problem. In many cases, replacing/cleaning the MAP/MAF and clearing the trouble codes from the ECU will mean no more P1100 appearing and you've solved the problem.

Author Jason Lancaster is a long-time Toyota enthusiast working with Olathe Toyota Parts Center, a website offering discounted OEM Toyota parts online. Jason also writes for Toyota Parts Blog, a website for Toyota owners.