Timing belts can’t be inspected for signs of wear, and can look good and fail shortly after. Timing belts will fail more often with vehicles that operate in extremely hot or cold climates, or in areas where the ozone levels are high. These factors will speed up the demise of the timing belt. Failure to replace the timing belt tensioner is likely to result in repeat failures. Replacement of adjacent components (e.g. water pump, balance shaft and/or injection pump; accessory drive belts) is advisable to minimize the cost of subsequent repairs. On this document also you can find information about suggested timing belts change intervals. If two symbols are used, the lower number should be followed for vehicles driven mostly in short trips (severe service). The higher number is for vehicles driven at least 10 miles per trip (normal service). The same is true for the double symbols.